This summer I put together a Shires large tenor by purchasing the parts used. It was a lot of fun, and the excitement of getting the parts sent to me was great, and I ended up with a MUCH better horn (for me) than what I was playing on. For context, I was on a Conn 88HCL with a 47-62 slide. It played great, but I wanted something new - I was looking for something that was more focused, had more firm slotting and attacks, while still having a warm rich sound. I REALLY wanted to not go too far on the smaller side, as I wanted something free blowing, with not much resistance; Conn was too free, and I needed something 'smaller' but still dark. I settled on a 7GM bell, standard Q rotor, (all that was available used at the time) standard Q tuning slide, and a TW47 slide. I guessed that the slide, rotor, and TS - all yellow - would yield a nice crispness that I craved out of my Conn, while the gold bell would maintain a level of warmth that I didn't want to give up.
I really loved getting the horn, and playing on it was fun, but it played just.. weird. There was more 'resistance' (a vague term, I know) than I was hoping there would be, and my mouthpiece at the time just really didn't sit right for me. I was on an Elliott XT 103, G, G9. The mouthpiece was PERFECT for my Conn, but didn't work at all on the new horn. I thought I made a massive mistake, and the worst part, I used up all my money. So I spent a month trying out different leadpipes (purchasing used leadpipes) thinking it would solve the problem, and I eventually settled on a 2.5 leadpipe - 3 was too large and led to a sort of bottleneck feeling (though it could pop out low notes) and it was so difficult to play - an air hog. So with the 2.5 in there, the horn was actually really great - I just didn't have the right mouthpiece for it. I spent some time mucking about buying cheap used Schilkes just to test things out. Being an Elliott user however, I knew that I definitively needed a 1.03" rim. What I had on hand was: Schilke 51D, 51C4, 52, (with a weird mod to the rim that made it a heavy paperweight) and a beat up 53. I had trouble practicing at all for a while - the DE was sooo out of place on the horn, but the second best thing was the 53, which only barely worked. Long story short, I got a Greg Black piece that worked perfectly and all is well.
I still want to tweak something about the setup though. I find it difficult to make long phrases on the horn, and I think maybe changing the TS or bell would be a subtle enough change to not push it too far, but also make it more efficient. I was thinking of trying out different tuning slides or maybe a different bell - of the many possibilities. Maybe not changing any diameters and trying a LW bell, thinned bell, or a different material would do the trick Or a different tuning slide, like the TX, or a seamed TY? Or maybe changing anything will set the horn off axis send me down another rabbit hole trying parts and generally annoying trombonists over the phone. I may be overthinking it (I am). I just can't help but see all of the possible combinations and think of the subtle changes that could result in swapping one part that would make the horn even better for me. I don't know, I just like messing with stuff.

Also sorry for the wall of text. I joined the forum not long ago mainly for the classifieds but have been checking it almost every day because.. I don't have many people to talk trombone with, and as sweet as she is for listening to me go on about trombone stuff, my girlfriend can only take so much.
Thanks for reading, fellow slide movers.