This is a great thread, I love seeing what people tossed out for whatever reason then changed their mind.
I keep a lot of stuff. I also find that I discover new sounds sometimes months or years later, with fresh ears and a few new settings. My TSL is the king of this, I loved it at first, then hated it, then loved it again after not playing it for like 3 years (including listing it for sale). Glad I didn't sell it.
Amps I have sold, then bought another of:
-Marshall Silver Jubilee 2555X. I got one, really didn't like it, recorded a quick video but then returned it. I got a lot of flak on the Marshallforum for not liking it, people saying the problem was I didn't use a Les Paul or a SSS Strat with it or whatever. Personally I didn't think that was fair but ultimately I still got bullied into buying another. I still do not like it, but I still have it at the moment but I'd sell it in a second if I didn't want to use it for another quick video (mostly to talk about LED diode clipping since it's really obvious on that amp).
-Orange Dual Dark 50. I got one of these for $1500 and then saw that they were selling for close to $3k. It was an alright amp but I didn't love it, listed it for sale, made a nice chunk of profit. Another one popped up for $1500 so I bought that. I figure I basically got a free amp out of that deal. I still have the second DD50, and I'm on the fence. It doesn't blow me away, and especially now that I have more Oranges I far prefer the Rocker 30 or OR50. But the DD50 does look cool AF and it gets really close to the same type of sounds as those two if you can be disciplined with the gain control, so it's staying for the moment. Plus, they are a bit hard to find, I sure wouldn't pay $3k for one but I'm hesitant to sell it and then regret it.
-Mesa Mark IV. Don't know why I ever sold this one, totally stupid. I thought the V would replace it, it does not, but I was tight on cash. Bought another IV and glad I did, will likely be buried with that one since it's super minty too. I'd be more likely to sell the V than the IV now.
-EVH 5150 III 50w, sort of, I sold the 50w to fund something else but later got the 100w (regular version). I really like the 100w but I remember liking the 50w a lot too. It was more of a sale of necessity than "I didn't like it." I do miss the Resonance control, maybe someday I'll get an S/EL34 model, but I don't find them to be different enough to merit the cost at least for now. On the other hand, I'm not afraid to make the modifications to convert my regular one to S specs, it's just a few caps/resistors/pots...
-Marshall JVM410H. Had one, felt my TSL was just as good in many ways (except the channel switching), sold it. Walked into my local Sam Ash and saw a used one with footswitch for $1000 and talked the guy into giving me the $150 off deal (which I think was for new gear only, oops!). Couldn't resist at that price so I'm sitting on it. It does some really cool gain stage switching and stuff so it'll be fun to talk about sometime.
Amps I sold and didn't replace, but would try again:
-Jet City JCA22. I did a few mods on this, switchable to SLO preamp spec, plus a few of my own values. I only sold this because it was a combo amp and I wanted heads. I'd buy a JCA50 or JCA100 for sure, but lately I'm thinking I'll just buy a real SLO100 or a really good SLO clone instead of a JCA.
-Hiwatt DR103. I went into this expecting something different I think, it was way too loud and clean for my old place. I can play much louder now, and I'd be more willing to try an attenuator with it, so I'd like to give it another go. The main issue is I have a mental block, where any amp over about $2k seems out of reach (yet I just talked about buying an SLO on paragraph ago so, idk).
-Laney AOR50 Series II. This amp had a very gritty distortion, super sloshy, not tight at all unless you reeled it in a bit and boosted. I actually really like the way it sounds recorded though, I have an old clip I uploaded for the guy I sold it to and I think it sounds awesome in that, but I didn't like it much "in the room." Side note, I'd really like to get this one again AND a 6-knob ProTube and compare them, maybe along side a 2203.
-Egnater Renegade. Played one of these when they first came out and thought it was super cool, finally got one like 10 years later (just forgot about it I guess) and really disliked it. Too smooth, too tubby, beautiful cleans though, just not what I remember. Anyway turns out the one I had was modified, and after I sold it I it blew a filter cap or something, the new owner replaced that and apparently loved it. So I'd like to try another one, in good condition and unmodified.
Amps I definitely don't like and wouldn't try again:
This could change of course, but I'm not particularly thrilled with my Blackstar S1046L6, ENGL Retro 50, ENGL InVader 150, Marshall Vintage Modern, Marshall JCM900 Dual Reverb, Mesa TC50, KSR Orthos II, PRS Sonzera, PWE EH3, or Orange Rockerverb Mk III. None of those are BAD amps by any means, but I think I could probably sell those and not really regret much. Worth mentioning that the Marshall JVM410, Marshall 2555X, and Orange DD50 also kind of fall into this category even though I bought those for a second try.
Of course, I'm saying this now, but I bet if I sold the KSR or Mesa TC50 I'd be buying another one in a year so idk. Hard to say. That's why I end up hoarding them all.
Oh, and I also sold a Mesa Dual Rectifier Roadster. I felt it was worse than the 3ch Rectos, and MUCH worse than a Rev G I had at the time. Can't think of a reason I'd buy another one, but it's too bad because I loved the concept. If I could have both channels of a Rev G plus two good clean channels, plus spring reverb, I'd be thrilled. Sadly that is not what the Roadster does.