I agree with others that the first approach should be NOS tubes for the preamp. You'll get the best bang for the buck with no risk of losing money.
But if you're into DIY projects, you can further improve the sound by doing some simple upgrades that don't cost much. I don't suggest doing this willy nilly. Get in touch with someone who knows what they're doing and has a good reputation. I highly recommend Jim McShane. He's well known at Audio Asylum in the Tube DIY forum and well respected for his amp upgrades.
jimmcshane@prodigy.net
http://pages.prodigy.net/jimmcshane/
He helped me tweak a pair of Antique Sound Labs for a ridiculously small amount of money (less than $200). We obviously didn't use boutique caps like Mundorf or V-caps, but the result has been very sweet. My top end is smoother, less fatiguing and more enjoyable. Anyone who knows what they're doing will need a schematic. Jim focuses on the power supply as much as the coupling caps and signal input caps. He's a fan of using film to bypass electrolytics, Vishay fast diodes in the power supply, Russian KY40 PIO for signal caps, Multicaps for coupling, and carbon comp resistors in the signal path.
Some will argue that it's never wise to tweak like this, but the fact is, most equipment was built using inexpensive parts to keep the price down. For me, a couple of factors dictate whether or not to go this route:
- I don't mind losing the money I put into parts. DIY upgrades rarely, if ever, increase the value of any equipment.
- Will I reduce the value of the equipment? It doesn't make sense to do this to an Atma-sphere, VAC, VTL, Canary, etc. In my case, ASL is largely considered bargain gear which is ripe for tweaking.
- I plan to keep the equipment for a long period of time. Why do it otherwise? There has to be a positive foundation to build on. You can't make a silk purse out of a sow's ear.
