ATC users

Oh hey! I have the SCM20SL Pros and P1, almost identical. P2 is the same amp topology as the P1 just more power. I think the P1 drives the SCM19/20 pretty perfectly as is. I’d get a sub, but probably one with built-in eq, like a JL Audio Fathom. The C1 is not such a value compared to the speakers and amp.

P2's do drive SCM 19s much better than a P1, I've done that demo many times as the ATC importer to the USA.  But a P2 vs P1 is not as dramatic a difference for the money vs a sub.  I'd recommend multiple subs at lower level, as opposed to one, if possible.  One causes the most room related [modal] issues. 2 is better, both near the 2 speakers but maybe not in the corners.  Huge bass boost from corners.  Try one away from one corner and the other a different distance from the other corner.   The smaller the room, the more dicey a single sub becomes in sounding good.  One idea is run the mains full range, run the sub low pass around 85-95Hz with a gentle 12dB /octave crossover slope.

Brad
Lone Mountain Audio- consumer
TransAudio Group- pro
ATC Importers to the trade 

I have no experience with the ATC amps but have owned the SCM19's for the past 3-4 months - great speakers, and I much prefer them to my previous Harbeth m30.1's.
Initially I was using a single Benchmark AHB2 (100W/8ohms) - which drove the 19's very well. Later I got a second AHB2 and run them in bridged mono (380W/8ohms). This gave a nice boost in performance in terms of ease, dynamic freedom and perhaps soundstaging/imaging - though the overall sound was (not surprisingly) similar. It's possible some of the improvement came from having a dedicated amp/power supply for each channel and shorter speaker cable runs made possible with the mono arrangement. In any case there was a very worthwhile improvement in my setup with the amp upgrade.

I own a REL S3 sub which I mainly use for HT and briefly experimented with the 19's, but preferred the overall coherence without the sub. What surprises me about the 19's is the taut dynamic power and depth of bass available from such a compact speaker. No doubt the sealed cabinet and massively built SL spec bass/mid driver have something to do with this. I really don't feel any pressing need to employ a sub woofer.