It is based off of a Wick 4AL. I also own a JP274 and e-mailed them to get the answer.And I'm playing on whatever mouthpiece it came with (anyone have a clue what it is or what model it is supposed to be like?). ....
On the subject.
I have played (owned) the Yamaha 321, King 2280, a '65 Besson New Standard, and now a JP274. I like my JP274 and it is the best euphonium I have owned.
It was comparable price to a non compensating four valve euphonium. I treat it like a Bb/F tenor trombone. It is there when I need it and if I need a low C/B I got it. . Once you put in a Doug Yeo bass trombone mouthpiece the low register really opens up.
I currently play a Wick 3AL which gives me enough of low and high register to make me happy with a rich tone.
The case for me was price to get a decent instrument new for the price of a used horn.