I have had both the C7s and the P3s. I do think the P3s require a bit more power and control from the amp than the C7s. In fact, one time as an experiment I tried a 6.5wpc single ended amp. The C7s sounded pretty good with it, at lower levels of course. But the P3s really couldn't be driven with it. The lower sensitivity and the sealed design probably combine to require more power from the amp. The Croft is very stable, but it is only 45wpc into 8 ohms and 50wpc into 4 ohms. These power rating will certainly place limits on ultimate volume, and perhaps that's what you're running up against.
Harbeth P3's or CS7 easier drive
I'm driving the P3's with a Croft phono Integrated. It's a very nice combo, and I would think very hard to beat for the money. Opinions may vary of course. I can't help but wonder if the C7's would unleash the Croft a bit. I remember the dealer playing the SHL5's for me after the P3's, and they easily sounded more efficient...Played louder with same amp and VC set the same. That was what I had heard about the 5's, so no surprise there. The C7's are spec'd at 86db, while the P3's I believe are 83.5db. However, the Harbeth site suggests an amp with higher wattage (than P3) for the C7's. Cheers -Don