Hi Bill, fun stuff for you.
According to this thread:
http://www.diyaudio.com/forums/tubes-valves/220403-moscode-600-tube-recommendations.htmlYour amp has a lot of tubes in it already. 6922s are very tasty, can be chosen to sweeten things nicely, if that is what you want. So you don't necessarily need to add more up front. I'm not a big fan of 6FQ7 follower, but I think there are subs for that, I used 12AU7 for that, you have to ask George what will work if you want to swap that.
Because your amp has tube input stage, it will have very high input impedance 100k+, so virtually any preamp source impedance will work OK. Some of the best sounding tube preamps have higherish output impedance (>600ohms,) but others in that category are very very low. Manley Jumbo Shrimp is only 50 ohms, so it can drive any amplifier with ease. I think there is one of those on Agon now. It is a warm but very nice sounding preamp, good features, good USA build, great service, great resale. The jumbo is clearer sounding than the original Shrimp.
If you want more transparency then you can use 9 pin tubes, if you want to sex it up with some phat 2nd and 3rd order harmonic distortion then look for octal (8pin) gain tube like 6SN7 family. 6H30 is a modern (1970s) Russian 9 pin tube made famous by BAT and Audio Research, it has excellent technical specs, sometimes called the "Supertube" but it depends on the circuit. In Moscode 402 amps I like 6922 better, but 6H30 sounds awesome in newer generation AR preamps like Ref 3 and later. Those are complex circuits so there is a lot more than the tube in play with AR.
I have always loved the octal sound, especially a 6922 gain with 6SN7 follower even better. Cary uses that combination to great effect, and they made a few mid priced preamps.
If I were in the market for preamp I would buy a deHavilland UltraVerve with remote. Member Clearsound in NY is selling a demo unit. Maybe you can audition that. I don't know if it has low output impedance, I might tend to doubt it.
When it comes to preamps, the best sounding units tend to cost more. SURPRISE!!! I hunted for a $1000 tube preamp once, and settled on EE Minimax. My Tutay modded unit sounded very good, very powerful and low distortion (20V into 600ohms!) but it made some EMI hum due to the small chassis size and it was a little thin sounding for my system at the time. I was craving 6SN7. Since then I have heard a lot more $1k tube preamps and have not been impressed.
I agree with Mike, an internal phono stage will compromise the phono stage and even the line stage to keep price down. If you do get a tube preamp, pick one that has a passion for excellent musical sound quality, not features or price.
Kara Chafee on feedback:
"Q: What is the story with zero feedback?
A: The Aries 845 amplifier uses no feedback loop or "negative feedback" at all. This is probably the most difficult way to design an amplifier, because the amp has to sound good just the way it is. Feedback was traditionally used to lower distortion and improve the tone of an amplifier-for example if you remove the feedback from most audio amplifiers, they sound "yelly" or have a tunnel kind of tone. With a carefully designed triode amplifier you spend endless hours matching and understanding the strengths and weaknesses of each component in the amplifier, until it sounds balanced and natural without adding feedback. The benefit then is that the amplifier sounds more open, immediate, dynamic and natural. Feedback tends to homogenize the sound."
http://www.dehavillandhifi.com/Q&Akara_chaffee.htmNelson Pass on feedback:
http://www.firstwatt.com/pdf/art_dist_fdbk.pdfIf the amplifier is designed to have low distortion and low impedance without feedback, then it will sound better without it. Negative feedback screws up the math, even a couple dB. But there are other kinds of correction that don't screw up the math, witness HCat, Spectron, paX, etc.
Line level tubes are usually linear enough to be relatively easy to design simple circuits without feedback and still sound good. A few dB of feedback around a stage might help it sound even better, or worse. All solid state preamps have a LOT of feedback, except maybe the diy stuff from Pass. An opamp has 100dB of feedback, even more than a Bryston power amp! You can hear it easily, even though it is .00001% THD, the infamous LM4562, the music killer. Preferred by test bench audio designers and reviewers and spec worshippers everywhere.