So. Current system is a Peachtree 220SE fed by a CD player, a Roon NUC, and a PSA NuWave Phono, feeding KEF LS50s and a KEF Kube 1 sub. Probable upgrade path when I can swing it is a Stellar Pre with Stellar M700s. Same speakers. Same sub.
Will the P3 be adequate for this setup?
Should be a perfect fit 
The P3 handles the Gain Cell DAC and M700s without breaking a sweat, and your CD player, Roon NUC, and NPC are all lower power consumption devices. The sub will probably need to stay on wall power because you’ll be out of outlets on the P3 with this gear hooked up.
Will it be able to manage the Peachtree in the meantime though?
It should be no problem with it on regenerated power, though if you’re pushing it hard enough that it’s more than the P3 can handle, you could just switch from regenerated to filtered - there’s a toggle switch on the back to change the HC outlet from regenerated to filtered.
I doubt very much that you would draw enough to come close to taxing the P3 with the Peachtree, unless you were running very large and inefficient speakers very very loudly. Maybe. My dual-mono nCore amp is similarly rated, and I don’t think I’ve ever cracked 100 watts running the Harbeth 3-ways (SHL5+).
Have always liked the sound of Peachtree stuff. Often one of the few rooms I give a mental thumbs-up to every year at Axpona.
That’s really the genesis of my question.
I LIKE my Peachtree. I like it a lot. While I’m seriously considering going to the Stellar line at some point, I may choose to stick with my Peachtree for many years and go for different speakers (magnepans with a bigger sub setup) but before that, I’m pursuing that next step up.
Yes, speakers. I get the argument. But, damn, I love the LS50.
I suppose I should just order the regenerator and test it for a few weeks and decide, but I feel guilty changing my mind if I don’t hear the difference.
Good man. But don’t feel guilty. Their decision to offer trials was carefully thought out.
Don’t ever feel guilty if our gear doesn’t work out in your home
We’d rather you return the gear on trial if it doesn’t do great things in your system.
If you do try it out, please keep us posted!
Be prepared to keep it ; )
That’s what I’m afraid of. I’ve never heard anything through a regenerator.
Most of us are impressed. Once in a while someone opines it makes little difference, or restricts dynamics, etc.
Please report back and let us know what you think after trying it out. 
Its like speaker cables - it ‘raises all boats’ in your system, and so may be a better investment than it might at first appear on paper.