This is based on my observations with a 2016 model. Newer models will have different transformers so the results may vary.
I would first look at the power tubes. If they are Mesa tubes, there should be a silver label on them, usually on the plastic base of the tube. On that label will be a three letter code. RED (obvious), YEL (yellow), GRN (green), GRY (gray) and BLU for blue. The red or yellow bias codes will have more headroom and less power tube distortion. JP2C does rely on that power tube saturation. Green or grey is what you should have in the amp. The Class A/B power section is different than the Simul-Class used in the Mark IV, V or VII that will drive that extended class A (push-pull) into saturation or distortion with ease. You do not have that with the JP2C so you need a power tube with early onset of distortion in the green, gray or even blue bias colors.
When the STR440 tubes went out of production, I had tried a few of the other offerings like the STR441, STR443, and eventually the STR445 and STR448 before settling on the Mesa STR415 NOS tubes that are not easy to get. the STR445 in green or the STR448 in gray bias colors were the closest I could get to the STR415 characteristic. The STR443 was a bit lame, they were ok I suppose but not great. I got those in the gray bias color. The STR441 tubes in green were very lame. I did not get any distortion from the amp with those, if I did, it was not very impressive. When it comes to the power tubes in a class A/B amp, the onset of distortion may make or break the full gain characteristics of the amp.
Side note: some amps are better served with the higher headroom in the red or yellow bias color ranges. Roadster is one prime example. Reds actually sounded the best. Most of the drive characteristic is from the preamp. MWDR on the other hand, the green or gray range was really good. Note this is based on the STR440 tubes. The other models STR441, STR443, STR445 and the STR448 may or may not behave all that great in those amps. The same can hold true for the JP2C. Depends on the tube impedance as well as the onset of early distortion. This amp seems to perform much better with more power tube distortion than the other amps like the Roadster or even the Mark VII.
A note on the alternate Mesa tubes: STR445 tubes vs the others, they are a very tight fit due to the slightly larger pins. They will fit into the socket, just takes a little patience to seat them properly. Those are the stock power tubes Mesa uses on the Mark VII and the MWDR.
STR448, tube length can be a problem. They are longer than the original STR440, which is about the same size as the STR415 tubes. I could not make use of the metal tube cage after I got them into the amp. Cage would not fit. they are difficult to install but it is possible if you angle the tube and get the pins aligned properly.
As for the popping noise, I was getting that with the original STR440 tubes. I first thought it was a preamp tube. Nope. It was the power tubes. That was the turning point when I got the alternatives, before I knew the STR440 tubes were discontinued. STR443 did curb some of the fizz and did not pop but had too much low end and lack of desired saturation effect. STR440 tubes were far better. That was when I first realized how much of the desired gain characteristic was also part of the power tube section. It is not all preamp tube distortion like many may believe. It is a Mark series amp, not a Rectifier amp. Dual or Triple rectifiers are more preamp distortion than power tube distortion. May not apply to all of the range of Recto amps. I only have two in my collection, Roadster and the MWDR. the MWDR is a bit different and you can get away with more power tube distortion but not the Roadster. It is just an opinion based on observations in finding the replacement for the STR440 tubes.
