Audio Research SP-15, either stock or GNSC?

Does anyone know much about this unit? I couldn't find a whole lot of info online. I am looking for a quiet tube pre at under $2500, or still possibly solid state. Anyone know what the GNSC mods due to this unit, and how much I would likely spend?

There are a couple of these stock for sale right now.
I bought an LS-15 about 2 years ago and had GNSC rebuild it. All I can say is that I didn't like it, stock, and was quite surprised with how much difference Steve Huntley's mods made. Steve is very serious about building the sound from the bottom up. It didn't have a very satisfactory bottom end stock, and when Steve got through with it, it did.

I don't know if this helps, but if you want to know more, feel free to email me off-line.
I have the SP-15 preamp. It is a hybrid unit. Seperate solid state power supply, MOSFET line stage and 3 ECC33 tubes in the phono stage. It also has one set of balanced outputs but has only SE inputs. The preamp gain is stage selectable in series with a volume knob. So for CD's I set it at min gain which gives a dead quiet noise floor. And at min gain the volume knob usually sits at the 10-11 o'clock position for my CD listening levels. For phono I have the gain set in the middle position for the Koetsu Black cartridge that I am using. That's a very low output cartridge so that shows just how much gain this pre-amp has. But note that I also have a pretty powerful amplifier. The phono section is very quiet too- especially since I need only set the stepped gain knob in the mid position. This preamp also has selectable cartridge loading settings which I find convenient- but really never change since I don't change cartridges. The owners manual has the specific gain values if interested.

The only mod done to my pre-amp is an upgraded power cord. The umbilical cord (between power supply and preamp) went bad on me a few years back. I sent the both chassis back to ARC since I didn't know it was just the cord. I had them replace all the electrolytics in the power supply section at that time based on their recommendation. Really, just because it was there and the cost wasn't too bad. I don't recall any real change in sound quality after getting it back.

I do not change equipment often- this is only my 2nd pre-amp in my hi-fi system. My first was an ARC SP-6B which I modified in 1989. The SP-15 seems to me to be fairly neutral with CD's and nicely musical with records. The sound stage with good records really opens up and extends well beyond the speakers and is deep front to back. Imaging is rounded (best way I know to describe it) which is typical with tubes. Listening to CD's the soundstage is smaller and not as deep. A few CD's extend beyond the speakers but none as much as records. CD's have a more pinpoint imaging- typical of SS. But in my system CD's are still rounded more than what I have heard on some high end SS systems in the past.

Compared to my old SP-6B this preamp has a lot more guts. Bass goes deep and CD's are more neutral/not colored. Phono sounds better than the SP-6B in every way as well- partly because the SP-6B could only amplify MM or high output MC Cartridges. That was one of the main reasons I changed preamps.

Here's my reference point on CD's. I have the ARC CD3 MKII. That replaced the ARC DAC3 MKII and transport that I used for 10 years. The DAC3 used ECC33 tubes while the CD3 is SS. As I recall, I expected a noticable sound improvement with the CD3 and it did not disappoint.

As for my comments about tubes and SS. When I hear live acoustic music- especially outdoors in the streets around Europe I often stop and close my eyes to compare the live imaging to my stereo hi fi. I find the instruments and voices live are more rounded like with tubes. So that always sounds more natural to me.
Tonywinsc - Very nicely said about tubes vs transisters. More rounded and natural.

It's awfully easy to forget that the real comparison should be with live music, not from one system or piece of gear to another.