Harbeth 30.1 or 40.1

@snackeyp dis you also upgrade the amp when upgrade to 40.1?  What amp did you use?

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.  

@gte357s 
My amp was a Lyngdorf TDAI3400 when I got the speakers but after that I tried an LFD NCSE 3 and prefer it.  It’s a solid state amp that sounds more like tubes.  Great amp, but very simple, and lacks the convenience features of most amps, most notably a remote control.  My second favorite amp with these speakers is the Primaluna EVO 400.  I might buy one in the future as a second amp because it’s really that good.  

@gte357sI ran a 4 sub array with my SHL5+40ths in a small room (10x12 minus closets).
The sub array really helped.
Now I am in a bigger listening room (14 x 17) and the SHL5+40ths really opened up at louder volumes.The sub array really helped in this room as well.

I have not heard the M40.1 or 2 but can vouch for the SHL5s.They fill the room without effort.
Seems like an even bigger room will let them play more into their "sweet spot".
I would strongly suggest considering the SHL5s and using the extra cash on a subwoofer solution.

@gt357s

mon 30 are midrange oriented monitor more for semi nearfield listening - will need bass reinforcement if run full range deliver dbs into 18x16 room

mon 40.1 are very full range, perhaps overly so... i had mon 40.1 before, they boomed, need to be far away from room boundaries - have you seen these speakers in the flesh?  they are huge boxes, know what u will be getting, if this is anything but a dedicated sound room there may be aesthetics issues

otoh, the c7 and shl5 bridge the gap between the two - c7 small box but more balanced high thru low, shl5 basically a larger version of c7 with supertweeter, similar voicing - 18x16 is good size but how will you set up the triangle, how close to room boundaries and therefore how much bass reinforcement?