By sound card do you mean the one your computer came with? If so you'll need a decent audio interface. If you have a decent interface with a preamp in it then an external preamp probably won't solve your problem with the bass. Most likely it's the amp that's rolling off the highs. I would consider getting a quality active direct box like a Countryman Type 85 and using that instead of or along with the amp and mic. Radial also makes some nice direct boxes. When one bass player I know first plugged into a Type 85 his jaw dropped and he said that was the sound he'd been trying to get for years. I don't know if it's what you're after but it's a pretty classic sound.

thanks for the reply tc electronic konnekt 8 is my audio interface. the sound is pretty good from the amp but only when i hear the recording the highs or presence is not there. setting on the audio interface and mic are set to flat response.

...i recently recorded bass by micing bass cab and even changed different mic positions the sound recorded was lacing treble frequencies although hearing directly through the cab the high end was set pretty high. but the recording did not get any high end. however this mic captures very good high end in vocals and recording accoustic guitars or even electric guitar through micing a cab. with the same setup.

2 things.

1. It doesn't matter what your settings on the cab say. What matters is what you hear.

2. What you hear with your ears in the room doesn't matter to the recording. It's what the mic hears and what ultimately comes out of your monitors.

When you're auditioning tones for a recording of an instrument cab it's a process of trial and error. But there are a few things you can do to help the process. For starters you can make certain that the cab is off the ground like on an amp stand or a stable stool or something like that. That way you won't get bass frequencies built up from the cab coupling with the floor. That can cause you to lose some treble.

Also there is the matter of keeping the cab out of the corner of the room. Centralize it so that you don't get bass frequencies built up from the corners as they naturally do.

Then there is the matter of where you're placing the mic. That's a big trial and error thing. But in general if you want a little more aggression and high frequency to cut through you could try aiming at the very center of the dust cap with the mic no more than an inch away. That is the most aggressive sound of the speaker. Ultimately you'll have to play with mic placement until you find what you're listening for.

How you audition matters as well. As I mentioned earlier it's not a matter of what you hear with your ears in the room. Mic the cab up. Twiddle the settings to what you think will work. Record a test take. Tell the bassist to stop playing. Listen back through your monitors. Does it sound like you want it to sound? If not then think about what it needs. Adjust the settings if need be. Adjust the mic placement if need be. Adjust the mic selection if need be. Then do another test take. Do this again and again until you get it right. The more you do it the more you'll have an idea of where to place the mics and which mics to use and how to twiddle the settings and all of that. There are so many things that you can adjust that it's a wide open experiment for you to find your tone. But that's the work of getting things to sound right at the source.

Don't be discouraged. There is NOTHING (short of possibly a different mic if all else fails) that you "need" to buy. Your gear is perfectly capable of getting a great sound. You need to improve your process for finding that sound though. It takes time and experimentation.

@ hey thanks mirrormix - i believe your suggestion is correct bcoz i did record the bass cab sitting on the floor while for the guitar cab i used a stool and didn't have any issues with the high end. next time i will do the same with the bass cab too. btw i still would like to see what should be my option if i want to upgrade my sound interface or preamps. if my budget is 2000$ US.
Thanks again.

ok let me explain my question again with a different perspective.
if i have the following audio recording chain. then which of them is a bottle neck or deminishes the true potential of the other gear in the chain and why ?

audio interface (konnekt 8)- 2000$ mic pre - 1000$ mic (shure ksm 44a)

how much improvement should i expect by adding a good mic pre worth around 2000$ in to my current setup which is konnekt 8 and shure ksm 44a mic. (bypassing my cards preamps through attenuators)

If your looking for an improvement, why not get a better interface with better instrument inputs and mic preamps.
As far as recording bass is concerned I don't think there is anyone who doesn't use a high quality DI for that and then add the amp cab into the mix. That is pretty much a standard approach. You have far more control over the bass sound directly. Using a good bass amp off the floor that has a great sound and speaker in a decent space with the proper mic, pattern and placement can be far more important than added gain from a $2000 preamp. Spending $2000 on a preamp sounds like preamp envy to me, but hey it's your money. You should find some reviews of preamps that fits the KSM44 and sounds good with that people have tried and liked. A $1000 mic like that can be pretty detailed, but for a bass amp I don't know if that's the best mic for that job....depending on how well it's setup it might be really nice!
Getting a good well played bass to sit in a mix and sound good is all about tight EQ around that bass sound and making sure is has that low frequency spot at 150-250Hz to bust through just above that kick drum...again like Mirrormix said...use your ears! You have the equipment.

my question is, would it be better if i get a good interface such as the xfx from RME or get a preamp such as NEVE 5015 for my current setup would i be able to get very good results, actually i have learned that good preamps give you more head room for better quality results and since i record only 1 thing at a time so i don't want to spend on a big interface. which normally in all audio interfaces as we go up in price more inputs and outputs are added. where as i am happy with two outputs/inputs.

If I were you I would listen to the advice you already have. Put the 2k$ in your bank account and concentrate on finding the best technique with what you have. The only exception to this is to get a good DI box as already suggested.

You will not solve this problem by throwing money at it. Learn the best technique to get nearest to the sound you want with what you already have, then and only then decide what is the best way to spend your money. By then you will have the advantage of what you have learned and be able to spend more wisely.

EDIT: BTW we all suffer from GAS when there are a few spare pennies around. Sadly GAS seems to frequently land me with gear I did not really need! (If you didn't know, GAS= Gear Aquisition Syndrome)

thanks every one for the help.