I recently bought a straight neckpipe for my Edward T-350, and gave it a first run last night...

The horn is definitely more responsive and "singing" with the straight neckpipe, is this a common phenomenon with convertible horns (seems plausible, more energy going into the bell), or is this possibly a function of a blob of solder in the valve section I stumbled across while servicing the valve section?

I found the same difference between an 8H and 88H. I’m sure someone would know the exact specifics of the reason. I would imagine it would have to do with multiple things like how well the rotor is sealing, the extra weight of the valve section…the extra bracing…

I’m guessing the phenomenon is the same in fixed horns vs modular ones…

I'm honestly not sure. I have a straight 42, a 42 with Ultra valve, and a 42 with two Olsen valves.

Sometimes I think the double valve 42 is by far the best playing of them.

I would not generalize it too much, but based on a few examples I would confirm it at least a little bit.

I used to have a German Voigt alto with a normal tuning slide and one with a Bb valve. The "straight" tuning slide played much easier.

Currently - as I had sort of tried to convince myself that I have a use for this kind of horen - I have a straight Blessing medium bore tenor and the same model, but with an F valve. The straight horn plays considerably more open.

Also, not exactly the same, single valve basses can have a degree of responsiveness that seems to be hard to get with dual valves.

And last but not least, one of my first trombone teachers/prof always played a straight Edwards lare bore during lessons. I don't know if he even owned a valve section at that point. I can only assume he did it for sound and response reasons.

Everything makes a difference, but it's not universal. On some horns removing the valve improves it, on other horns adding a valve improves it. On other horns it seems to make no difference, like my matching 1970 3B and 3BF that play exactly the same.