I've been asked to cover bass bone in a jazz group and my Olds S23 is tightly wrapped making the lower range a bit challenging with my current mouthpiece (Yamaha 59L). I'm looking for mouthpiece recommendations to open up the bottom end.

Is your olds well-maintained? When is the last time you had the whole thing chem cleaned? mineral deposits and such on the inside can make it feel less responsive. Also making sure the valves are well-aligned is key.

How low of a low range are you talking about? A Yamaha 59 is between a Bach 1 1/2G and Bach 1 1/4G in terms of throat size and rim size, though closer to the 1 1/2G. However, I should say that practice helped my low range on bass more than any mouthpiece change.

Normally a Yamaha 59 is big enough for most basstrombonists on Olds S23 in jazz. The wrap is usually no problem if it is clean. Check that the valves are aligned, clean the tubing. Maybe try a Yamaha 60, or Bach 1 1/4 G. Practis can make wonders.

I don't know that a bigger mouthpiece is going to be much help; you'd still be blowing through that .554" upper tube, the small rotors, and (depending on the vintage of your S-23), the .565" attachment tubing.

I regularly play on a pair of late 1940's S-23's; the trigger register is there, but it's a lot more work than on a modern bass trombone, even one with conventional rotors.

How well does the Yamaha mouthpiece fit the receiver? Earlier Olds bass trombones have a significantly undersize receiver; that can cause issues with response, particularly in the low register.

The best advice I can offer (beyond what's already been presented) is to make sure your slide positions are dead on for the trigger notes. If you're lipping the notes, even a little bit, the response will suffer.

I had an Olds S20(?) that I put a Duo Gravis valveset on (spec-wise, not much different from your S23) . That thing really rocked with a Ferguson L. A heavier blank can make a big difference.