I've heard audio samples of the modded (not just the tube!) and they sounded like a simple, cleanish circuit. If you're REALLY on a budget, this might not be a bad option, but I'll stress again, that there are problems with the stock units in the gain section, which has little to do with the tube.
Stock, my experience with two MP's before I knew any better, was that they were unusably noisy and poorly built. These unit were purchased new about seven years ago. A couple years late I bought a rackmount ART "tube" pre and while it was more solidly build and quieter than the MP's I used, they weren't in the same league as
a JoeMeek 3Q and the pres on my M-Audio Delta. They exhibited a lack of detail and muddiness in the lower mids.
It's odd that the reason a lot of people consider buying the ART preamps is because of the tube, but with the exception of the MPA (which still has solid state input and output buffers) the tubes are implemented in ways that have little to do with real tube circuits. I wanted these things to impart tube magic to my stuff and fell for this logic (3 times!), but learned my lesson. Seriously, if you've got to have a TUBE pre, save your money for a few months and buy a Brick or a used
Solo610.
That said, I'm not a hater of all ART products. The stereo compressor mentioned earlier is pretty useful, and the vari-mu compressor in the Pro Channel is a fun effect.
Craig