ATC users

I run ATC scm 19’s with a P1 pro in a nearfield setup. Any thoughts on which of these two upgrades. A: upgrade the amp to the P2 B: add a C1 subwoofer.?
the sub whould be nearfield as well, just in front of left speaker and about 4-5 ft from listening position.
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.

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.
I am very interested in your opinion on Benchmark AHB2 to drive SCM19. Do you think it is a good investment for 2 AHB2? My room is not that big (I would call it nearfield), and I personally do not like to play music at a high level. So is it necessary for the second AHB2? Thanks in advance.
I defer to tobes’ positive experiences above, however, the cost of 2 AHB2s + SCM19s is higher than a pair of fully active SCM19As and getting close to SCM40As, too, so wouldn’t think passive biamping is the ideal thing in comparison.
I agree with tobes assessment of AHB2 with ATC SCM 19 speakers. I am running the same combo and couldn't be happier. At one point I did get interested in trying out an amp with more power (I tried Parasound A21 and Cambridge 851W) but couldn't get the same tonal balance & accuracy as I am getting with my one AHB2 (stereo). This amp is only 100 watts, but due to its' 'no distortion' circuitry, all the wattage can be used. You need to listen to it to believe it. Whilst there is no need to add additional sub, I did try adding a sub-woofer to the combo, but since the tonal balance messed up badly (bass was out of control/sync in most of the tracks), I took it out.  I have listened to many hifi combos, but I keep coming to ATC + AHB2 for my needs.  All the best.
I am running the P2 with the 19s and in my smallish room there is no trouble getting punch out of the 19s. I have a C1 In the box and the CA2 preamp has sub out but I just did not feel I needed to even bother testing it.

only thing I would do over is just buy the 19As and bag the amp completely.