Amp for Harbeth p3esr

Your current amp should work wonderfully in the situation you've described.

I second the Belles Aria. Had one before and it worked great and I have tried many! Liked it so much my second one arrives Monday. 

I use the P3ESR with a refurbished Quad 405-2 and that is fine (the sound is gloriously neutral and insightful, to be precise), but so would be your Sony. Harbeth speakers are designed to sound their best with any properly designed amplifier, and their designer himself has a challenge out that you could never tell the difference under properly controled conditions (unless the amp is badly designed, of course). So just don't worry and don't allow yourself to be talked into wasting money on fancy electronics.
Of course, output in the lower frequencies is restricted by the laws of physics, even if this little speaker pushes the limits. I don't think that in a small room like yours you would ever want more bass, to be honest. I am using mine in a study about one and a half times the size of your room, and I don't think I would want more bass even in that room (I hate room modes and the woolly and boomy sound that they excite). If you ever move into something larger, you could always add a subwoofer connected at speaker level - you do not need a separate sub out. There is nothing wrong with connecting at speaker level, and some (e.g. REL) will even argue that it is preferable. In fact, that is exactly what I did by way of experiment. I tried the P3ESRs in my main system in a much much larger room, with a B&W PV1d subwoofer tamed by a DSpeaker Antimode 8033 dsp room eq (and a Quad 606-2 amplifier). The sound was spectacular, with deep bass seemingly coming out of these little boxes, and integrated to perfection. The only limitation was the size of the sound bubble and dynamics. If you really want to fill a big room, you need a bigger speaker. But in a room of say, 15x20 feet, or perhaps even a bit bigger, a pair of P3ESRs with one or preferably two smallish subs and room eq will be glorious.

Listening now to my P3ESRs < Tellurium Q Black < Belles Aria < Chord Anthem 2 RCA-RCA < Chord 2Qute < Chord Signature Tuned Array USB < microRendu.

One word, Glorious! Midrange to die for. Plenty of bass for me. Belles really does give you tube like highs and most of Naim’s PRAT. Maybe I could say its more natural. Have had so many integrated amps. Simaudio and Croft were probably the best. Heed wasn’t bad but didn’t have any guts and hated the volume control. Had reliability issues with LFD. I could go on and on. Oh, Belles stays nice and cool. One person said it’s like Pass Labs without the heat.

Bass is so good. Right now I have a JL Audio Dominion d108 and a REL T-Zero. Don’t even think I’m going to hook them up.

I agree that these are magical speakers - I love mine. In my study I use them without a sub, but I did try them out with a sub in the large living room, and that worked very well. So depending on the size of your listening room you may well enjoy adding a sub . I did use an Antimode 8033 room eq, however. Crossover at 45 Hz and 12 dB slope for best results.