I play several Eb horns. For the larger horns, try the Sellsmanberger Symphony with a 1A or 2A rim. (32.6mm) Another choice, if you like larger rims and generally a deeper cup, the Stofer Geib is good.
I have found that a Marty Erickson ME-1 works he best for me on my Besson 983. The Bloke imperial also works almost as well and I use a Lexan rim on it when I have to do a ton of playing (5-6 hours a day).
I've been using a Blokepiece Solo (the one with the bierstein) with a #1 rim(33.2 mm). Lately I've been using it with the spacer. I like it a lot. I don't know where you're located but trying some out is really the best idea if that's possible. Although my YEB-321 isn't a 5/4, I have tried it (briefly) in Norwegian Star and it seemed to work fine as did the Imperial with the same rim.
Good Luck
Pete
ppalan
Mirafone186 CC 4v
Yamaha Eb 321
Wessex "Berg" F
Do you want a bright tone, dark, or something in the middle? Do you want more of a bass tuba sound, or do you want to sound more like a contrabass? Or something else entirely?
JP/Sterling 377 compensating Eb; Warburton "The Grail" T.G.4, RM-9 7.8, Yamaha 66D4; for sale > 1914 Conn Monster Eb (my avatar), ca. 1905 Fillmore Bros 1/4-size Eb, Bach 42B trombone
For years I used a PT50 with an enlarged backbore on my Besson 981. Fantastic fulness and depth of sound, but very demanding in pp staccato. For me the main problem was the wide rounded rim with almost no edge.
The original blokepiece withe the #2 rim gave better technical facility, but gave less warmth to the sound.
The best mouthpiece I have for that large Eb tuba is the Mike Finn 3H that I had bought for my big BBb basses (where the aperture wasn't big enough for me). With the 981 it gives a big sound with shine and projection.
Blokepiece Imperial, and get a spacer to go along with it so you can try it both ways. After my last post, I have been woodshedding to get ready for the next season of community band, and the more I play the Imperial, the more I like it for just about everything.