TubeNet

There is generally no inherently correct match of horns and mouthpieces. Buy the moughpiece that works the way you want it to work with your face to get the sound you want -- which means you have to do a lot of test driving. You might describe what you would like to change about the sound you are currently getting, then any suggestions you get stand a better chance of being helpful. If you can list mouthpieces that didn't work for you and why they didn't work, that would help too.

If this is my old horn, I tried a bunch of pieces on it. Some worked great and some didn't. For your aplication I would strongly suggest a first generation Arnold Jacobs mouthpiece, produced by Terry Warburton and for a while marketed by the Canadian Brass. The infamous Warburton Neilan mouthpiece was actually designed for that horn and was based on an AJ with more mass and a C4 style rim.
Lee Stofer tended to like a Bach 7 (I originally mistyped 9) on that 1290 for a large Alexish sound.
About the smallest you can go on that horn for quintets or solos is somethine like a Yamaha Jim Self. Anything shallower or tighter, like a C4 or even a stock C3, just didn't work on it for me.
P.S. Since the 1290 does have some similarities to the 1291/1292, you may want to try a G&W Alan Baer CC (Euro Shank) mouthpiece on it.

I see that your tag says EMU so I am assuming you are at Eastern Michigan, and if you are, take a trip and visit Keith Powers (I believe that is his name) at Custom, they will let you play on as many as you want till you find the one that is Best for you. No one mouthpiece works for a horn, it is the combination between what is comfortable for you and what sounds right on the horn.

Bernie "I never played on a mouthpiece I didn't like" Williams