The room I use is my home office 8x6 feet where I work (technical/art illlustration) so it's not designed for music,... There are lots of shelves around the room , two desks, filing cab and drawing table. ... I was wondering if a top quality pair of headphones would also help with mixing ?
Dude...6x8 with lots of shelves around the room, desks, filing cabs and a drawing table!? Either you are a very tiny man, you don't mind not moving for extended periods of time, or your measurements are slightly off. In any case, yes this room is not well suited for music production, but that doesn't mean you can't make a good mix in it.
Headphones aren't really a good solution for mixing on a permanent basis. They have a tendancy to sensationalize your mix. Plus, if you spend a lot of time with them on, you will eventually damage your hearing.
Kurt is right, your monitors and pre's are your biggest weakness. For a small room, you need a monitor designed for extremely close up work. I know of only two that will work in this small of an environment - Blue Sky Media Desk and NHT Pro M-00.
As for the pre, just about any pre would be a better choice. If you are looking at upgrading the soundcard, perhaps you can go with one of the offerings from MOTU or Presonus that offer sound card and pre's all in one. They aren't the best pre's but they are far better than your run of the mill $100 preamps. Actually, the Tascam FW-1884 has pretty good pres for the money and acts as a Firewire audio interface that works as a great control surface for Cubase SX.
The simple fact is, for your set-up, you aren't going to reap the benefits of a Avalon or Millennia pre and Dynaudio monitors just won't help either. You will get very good results out of mid-level equipment if used the correct way.
I don't wish to discourage but there are times when the best advice is don't bother.
The dimensions of that room are such, you are never going to get accurate bass in that room no matter what you do .. You could line all the walls with bass traps (of any design) and I suspect when you measured it you would still see serious anomalies.
So at this point I think the best you can do is to learn to mix and record with what you have and leave it at that. At some point, when you can get into a larger room, the expense of better pres and monitors, as well as room treatments, can be justified.