Help me complete my stereo system (Harbeth, ClearAudio Concept).

The p3 is a good choice for that size room! This guy likes them as much as I do!
https://www.youtube.com/watch?v=mqFIaiPT_kY

Speakers, without a doubt are the most important choice, they will be the ’sound’ of your system. Everything else will be small refinements, heard or imagined.

You may love the sound of those speakers, however I advise you to keep on looking before you buy them.

Think long: you may move, you may want to try tube amps, you might want to try bi-amp.

Try to find more efficient speakers you also love. As primary speakers, I also would try for a larger woofer, I prefer no ports, if so, front ones only.

If you do buy small woofers, perhaps think about adding a pair of self powered subs now or later. A pair, front firing, located near the mains. Bass, fundamentals and the bass overtones do produce stereo imaging, a single sub dis-allows that. Bass players in jazz are often a significant part of the group’s success, and enjoyment.

Letting the self-powered sub take the low bass, sending only upper bass, mids, highs, makes it much easier for your amp, and, critically, the smaller woofers no longer need to try to make low bass, that's a big deal when using smaller woofers.

Efficient speakers works itself into the system, gives you many more options. Most importantly the more competitive world of lower powered amp(s): smaller physical size; weight, heat, placement options.

Lower cost amp(s) allows more for other parts within the budget of the initial system, i.e. more efficient speakers, less amp cost, gives more money for the speakers themselves, or more for the preamp.

I too listen to a lot of Jazz, stereo and mono. I recommend choosing a preamp with a ’loudness’ circuit, automatically/progressively engaged only at low volumes. Maintaining the bass player at low volumes, for me, retains involvement with the music, loss of the bass player becomes background music.

Recording techniques prior to Stereo were quite good, many great bass players. Not the location, but the distinction of individual players is refined by using several techniques: mono mode; mono cartridge; single speaker. Even in Mono, maintaining a more distinct awareness of the bass player at low volume retains involvement.

https://en.wikipedia.org/wiki/Equal-loudness_contour

Yes, if you buy less efficient speakers, you will need a V-8. Keep in mind, you need twice the power to produce a perceptible volume increase of +3db. Choosing 87 efficiency means you start out needing twice the power to produce the sound level of a 90 efficiency speaker. Or, choose a 90 db efficiency, you need only half the power to produce the same sound level.

A bit more, double the amp power again. It adds up.

Yogi,
I've watched that review so many times lately, I find it relaxing for some reason, he makes a great case for the P3s sonic signature and what is so attractive about them.    

Elliott,
Thank you for your comments/recommendations.  These are all valid points and many of which I've already considered.  Same as mentioned in the De Filios video, I'm not inclined to add a sub. If I feel the bass would not be well served, that would mean going to a bigger woofer, hence why I also have C7s under consideration.

The dilemma with P3esr seems to be that, while they seem like a good bargain, they can't be exploited to their full potential with budget tier amplification. I think the P3s would do a more than adequate job for my space and budget.  If I move and have a larger listening space a few years down the line, I would expect the P3 setup to become either a secondary system or make one of my sons very happy. My budget would move within the reaches of HL5 or 30.1 or the like.