I don't know what caps are in the Select. My experience with Jupiter coupling caps is they are comparatively quite good at first, with less burnin issues, and getting great with time. Variable periods of better versus less great sound are theoretically a lot about "seating" the caps. By warming up fully the internal metal settles in with the dialectic, and then by cooling, seating finds a more complete place. This is I think is a lot of what Steve's 5 hours on and 5 off thing is about.
I have heard it said that beeswax is transparent as a dielectric, and with silver wire, there is little burnin required. But on the deeper subtler levels, there is no doubt that Jupiters need burnin, especially noticeable if your system/room is revealing.
With a new amp, there are other burnin factors...think about how speaker wires and ICs need burnin, even ones built with minimal dialectic and transparent material. A new amp has wire and connections as well as power caps and transformers, not to mention tubes.... but the coupling caps position and contribution to the signal sound seem extra important. I have used new Jupiters in amps that were very well seasoned otherwise, and find Jupiters can take several hundred hours to even and clear up, and may not be fully refined until 5-600 hours.
Luckily, they sound better on the way than many..including less ups and downs.
This is mainly what I hear during burnin:
Highs are harder/edgier, and with time become more complex and real. This shows with more sense of resolution...the sounds seeming to be made of more "particles," feathering edges with more complexity. As highs refine it also shows more in ambient presentation...more room/player character, complexity and clarity.
Mids can be a little harder also, congested at times, and open at times. They also increase in complexity with time becoming clearer and more solid, richer from more texture, speed and resolution.
Bass is often the slowest to fully resolve. Also variable for a while, it will become more resolving, defined, and rich with time...tighter with more sonic information, more tonal depth and body.
Each of these effects the others.
The variability is the most disconcerting burnin thing for me and seems to be particular to amps. At times I will think, wow, this sounds great! And next session it might be duller/thicker, more congested, less defined...Or one frequency area can be pretty good and others less good, making the whole sound off. These things are recording dependent also, some mixes accentuating the issues, but the wildcard of burnin "ups and downs" can be a disconcerting experience no matter how you experience it.
I replaced Jupiter HTs in my MKIV several weeks ago with Jupiter Coppers. I finally got a Frybaby burnin device after too many years of burning in cables, caps and new tubes left me dreading the degradation of sound during burnin. I ran the Coppers in the Frybaby for several weeks before installing them, and they sounded better in many ways than the burned in HTs right off. For several weeks they have been a little up and down, but recently things have evened up to a better level...very good. I expect more with more time, but all is well and it was quite good for the most part before now. Can't say for sure, but this makes me think the Frybaby is good! It has really helped on power cables and ICs also.