I recently picked up an Aguilar DB659, which means I now have a decent amount of sonic ground I can cover along with the Demeter VTBP-201 I already had. Now that I have these, I'm kind of interested in swapping the tubes for some others and seeing what I can do (if anything) to make them sound nicer. It's not that I dislike the sound of either, but I'm a tinkerer and have had fantastic luck in the past doing tube swaps with other gear. I don't have any idea what tubes either preamp has in it or what condition they're in, so it certainly can't hurt. Has anyone done anything like this with these two preamps in particular? If so, what tubes would you recommend? I've been doing some searching and found Tom's tube swapping post which is a good starting point. I am hoping to get a decent assortment to play with, but there are more types out there than I'm willing to buy so any advice would be appreciated.