Harbeth 30.1 or 40.1

You'll always be able to find a used pair of 30.1's or 30.2's, but 40.1's come available only a few times a year... and local availability is a huge plus as you avoid shipping costs.  If you get the 30.1's you'll likely be content, but also always second guessing how much better your sound could have been had you bought the Harbeth flagship model 40.1.   Low to medium volume jazz and vocals mimics my listening habits and I've yet to own another speaker that comes close performance wise to the 40.1's, including stints with Dynaudio Confidence C2's, C4's, Daedalus Ulysses and Harbeth SHL5's with a REL sub.  Also consider many giving their opinions to your post have never owned 40.1's... or if they did, maybe their listening habits/musical tastes don't mirror yours.

If you choose to go with a solid state amp, the Plinius SA 102 gave me the most satisfaction of the solid state amps I tried with my 40.1's.  There's one recently listed (Listing ID: lisa4409) at a reasonable price that was recently recapped.  I still prefer tubes with my 40.1's (Music Reference RM9 MK2, McIntosh MC275 MKV, VAC 70/70)... but the Plinius was still good... just not a s good as the aforementioned tube amps.  If you are a tube guy as you say, you won't be satisfied with SS,IMO.

Long time 40.1 owner here.  Your room size is fine for 40.1's... and no, you don't need a powerful amp... especially for listening to jazz and vocals at moderate levels.  A good quality amp (I prefer tubes) with 75 WPC is all you need.  I've also owned Harbeth SHL5's... but they don't compete with 40.1's in my experience.  

Agree with @twoleftears , an 18' x 16' room is far from small and can accommodate at least 2-3 feet of clearance that most speakers need from surrounding walls to sound their best.  Based upon the listening preferences the OP described in his initial post, I'd be shocked if the 40.1's don't exceed his expectations.