It seems that most amps, regardless of the brand, have their own personality. I've A/B'ed many of the same amps in stores, and each one sounded or felt slightly different.
I bought a Solo 50 Combo Cabinet on Ebay for $100. I recovered it in the tolex of my choice (Wine Buggy Whip Taurus), and swapped the parts from my Rectoverb into it.
I absolutely love the amp, it was worth it. I've tried a few other Rectoverbs, but they don't seem to have the same feel & tone as this one.
I think it goes beyond tubes. Anyone can say "Oh, it's just the differences in tubes," but there are many more things going on in an amp. It could be that parts were substituted (Marshall & Fender do this alot), or maybe SBE had a bad day at the Capacitor Factory and made some shitty, or better, capacitors.
If you like your amps, and especially if they are out of production, you should be able to customize them in any way you want. I've built a small business based on this premise. Many of my customers want a unique sound or look for their amps, I am only too happy to oblige, and I don't charge an arm & a leg.