I "somewhat" agree with Camadeco as you might have summed up from my first post. Bargain hunters ( most everybody on this site ) see the 16 bit Alpha's for sale at reasonable prices and have a hard time spending the greater amount of cash for the 24 bit. As such, it may be harder to move them at a price that takes into account the cost of the upgrade.
In terms of tubes, there are literally dozens and dozens out there. What tubes work best in your system is going to be up to personal taste and the tonal balance / amount of detail that your after. My suggestion is to "start cheap" and work your way up. It is quite possible that you can find something that you like for low bucks and it MAY end your search. Then again, there are those folks that are "never satisfied"... : )
There was a similar thread to this one a few weeks back. I think that Kevin Deal at Upscale Audio said that the Mullard's were what he sold the most of for this piece. Then again, i think that people have read that others were using them and tried that as a starting point since they already knew that they would be decent. Kind of a "monkee see, monkee do" type of events. No offense to anyone on that statement, as i too was a "monkee" : )
You might want to try looking at www.partsexpress.com to see what they have. These are new tubes and they are an electronics supplier. For the price of one "big name" set of tubes, you can literally purchase a wide variety and see what works best for you. For your convenience and for those that may be interested, i've included some part numbers below:
072 - 410 Sovtek 12AX7WA / 7025 LOW NOISE $5.25 EACH
072 - 420 Sovtek 12AX7WB / 7025 HIGH GAIN $5.90 EACH
072 - 662 Philips 12AX7WA $10.80 EACH
072 - 680 G.E. 5751 $11.50 EACH