Perhaps a little description of what kind of music you are recording would help us to give you some advice? Otherwise... the following may help to clear your mind or clear your sound or maybe just the opposite?

The Millennia is so far removed from the UA 610, you're asking about the difference between apples and oranges in its sonic signature. The Millennia is considered to be extremely sonically transparent. Whereas, the UA 610 is loaded with calories, fat and color and is the epitome of old-school versus new school. So we can't tell you which hand to wipe yourself with?

I don't like any of this new crap.
Mx. Remy Ann David

Thanks for the reply. I record a wide range of styles of music from Jazz; Trio to Bigband, Rock; Sparse Acoustic to Medium Hard, Gospel; Small Band to Full Choir, Funk; Organ Trio to 11 piece Tower Of Power.

I guess my question is if going for something like the Millennia TD-1 is simply a different tone or better since it costs pretty much double the price of the 610?

I'd still go for the 610 if I were you but I'm not. It has a tone and a thickness that the Millennia I feel lacks. The Millennia is that esoteric reference style, almost instrumentation style preamp. The 610 made lots of rock 'n roll, funk, gospel famous. And I love the Tower Of Power since I like to do the Oakland Stroke, yeah. Plus I've got all of this Detroit in my blood. And this was that Bill Putnam design that made so many people famous. So don't waste your money. I mean it's not a waste if that's the sound that you want, that technical purity, transparent, blah blah. I like a preamp with balls and that's got more than its share.

I only do API & Neve, Altec Lansing 1367 and I'm happy.
Mx. Remy Ann David

Thanks Remy. I noticed that the TD-1 can do solid state or tube on the DI so am wondering if I could get the best of both worlds with it? Maybe the tube di output of the TD-1 is still very clean compared to the TD-1?
I've read a few top engineers claiming that the TD-1 is the very best bass di but then I guess personal taste can differ!?

I used the TD-1 when it first came out, and I still have my Solo 610. I know this sounds a bit weird, but the TD-1 PISSED me off!! All of those little knobs and switches scrunched together on the side of that compact box, with hard-to-read nomenclature. I used it for live remotes, up on the stage, for bass/keys/guitar (depending on the type of band). YOU try to read that little panel in a dimly-lit setting...Uggh! For that, it was overkill. I did like it with a 4047 on honey-voiced chickies in big band ensembles, usually on the SS side, but I kept looking at it and asking myself, "How much did you pay for that thing?". I sold it to get a new set of sidedrafts for my TR-6...that thing pissed me off, too...now they're both gone!
Remy hit the nail on the head, the TD-1 is very high build quality, and Millennia makes some nice preamps for precision recording of classical performances, etc.
I have lusted for The Origin after hearing some VO's I did that were recorded through it. But for bass in a horn band? My nickname for the Solo is "The Fat Bastard".
There's nothing in the world like bangin' a honey-voiced chickie on the fender of a TR-6 idling at 1200...with a plug wire pulled!!!! Trust me on that one.

I'm wondering if the Millennia TD-1 may be a better replacement for the ISA One though I like this preamp? I also don't want to lose 2 channels of high quality A/D conversion.