This box has lots of lovers and lots of haters. I think I have figured out why.
First off, for a single-channel preamp, it has A LOT of ways to hook up - it has a solid state input and a tube output (full voltage - this is Summit, afterall). HOWEVER, if you don't want to use the tube, you can take an output that keeps it solid state. You can also run two different signals at the same time, one through its mic input and one through it's DI. So if you have a ribbon mic that needs no phantom power and a condenser mic, you can play with mic placement on an acoustic guitar and have it come out of several outputs (all in mono however). It's wildly flexible, but maddening once you start experimenting.
It has an impedance knob, which helps color the mic signal. It is listed as variable, but in reality, it's either "on" or "off". It has a -20 pad, +48 phantom power and a great HP filter. The sound of the preamp is big, so that high pass filter becomes your friend very quickly.
The output gain knob is where the tube lives. This unit brings out the sparkle in a tube - and the better the tube you use (the one that comes with the unit is a Ruby and it's pretty bland), the better the sparkle. Old RCA's or Valvos are my favs in this. You can literally dial in how much tube flavor you want in the unit, although the more tube you bring in, the noisier the unit gets (not bad tho) and the transients begin a very slight slurring as the tube takes over.
People often complain about noise and the tone being somewhat metallic. I have discovered why, after using this for four years. I was bored one night and decided to try it in ways I never had - I turned the impedance knob all the way to the left (essentially defeating it) and turned the -20 pad ON. Suddenly, the unit sounded like the "grown-up" Summits I have used before. Still big, but now the roundness was there, without a spitty top. And the noise level was far below the comparable level of the unit as I used to use it. The metallic edge was gone too. This is a great vocal unit now.
You can sometimes find these units used for $400-500. If you treat the "-20 pad" as a "+20 pad" and ignore the impedance knob on everything except muddy sounding mics, you will be really, REALLY happy with what this unit gives you.
And for clean guitar tones (bass too), the 2BA-221 really excels. The DI is rounder than the mic input, but I'll try both, with differing but satisfying results. The mic input gives you all the controls I listed (and THIS is where the impedance knob and "+20 pad" come in handy), however the DI is just a straight input, which you can color with the tube gain output. I sometimes use the unit to re-amp synth'd pianos with warm results.
So there ya go. It's a great compliment to a GAP73 and if you score one used, REALLY affordable for the quality you get. And it's built to last.