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TheHatTuba wrote:What might be the cause of a larger bore horn having a worse low range?

You. Horns act differently for different people. The few RMs I've played have had excellent low registers. When I played a 56J, when I started playing CC, I never got comfortable with the low range. That changed when I played another one a few years later.

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The bore on the RM is 18 mm (.709 inches) which really is only .5 mm larger than the first three valves your Conn. The fourth valve tubing (.730 inches) on your Conn and your fifth rotor (.750 inches) are actually larger than the RM's straight bore of 18mm. So the RM has a marginally larger bore except when it doesn't.

The lead pipe on the RM starts out much much larger than the one on your Conn. Your Conn also has a separate receiver, where the RM does not. As a result, your Conn probably responds quicker and articulates quicker. One has to use a very hard articulation with a RM tuba because it responds slower. This is why many folks consider RM tubas to be "harder to play" than other instruments. However, also because of its very large lead pipe, the RM tuba is likely easier to play louder than your Conn.

I would expect that the pitch on that RM to be better than the Conn.

I've owned one of these RM 4345 tubas for 6 or 7 years. I had a RM 45 CC for a number of years previously. I have played several of those Conns. I do quite like them, but favor the 52j over the other two bell sizes. I hope to someday to test changing out the lead pipe on my RM 4345 CC to a lead pipe that starts out smaller and has a receiver. I think this will improve the articulation and quicken the response of the low register. For me, the low register has resistance, but I've learned to do it. The advantages of this model are sound and its "better than Miraphone 188" intonation. The intonation of the RM 4345 is as good as tuba intonation gets.

It does seem to need a mouthpiece that works with it. The Schilke Helleberg II which I was using prior to buying this tuba did not work very well with it at all. The low register also seems to work better with an American shank mouthpiece than a euro one. The horn also likes a large mouthpiece. I use an American shank Sellmansberger Symphony on mine. The Sellmansberger Symphony has a super fast response which helps out with the typical "not so fast" RM response. If I weren't using that, I'd probably use a G&W Taku on it. I have also previously found the following pieces to work OK with it: Conn Geib, Stofer Geib, Sidey SSH, and Conn Helleberg.

I hope this helps,

Christian