Harbeth 30.1 or 40.1


I would like to get a pair of Harbeth, and wondering if 30.1 or 40.1 is better.  My room size is about 18’ x 16’.  I currently have a pair of Spatial Audio X5 with large 12” mid range driver.  I would say I listen in low to low-medium volume for Jazz and vocals.  Is 40.1 too big for my area?  I am afraid if I am not listening to what it is supposed to sound like if I don’t turn up the volume.

however, if I buy the 30.1, is that I should get a pair of sub?  Versus for 40.1, I assumed I don’t need subs?
gte357s
@jtcf  thanks.  Yes, I am not planning to buy it immediately, but be prepared when something comes up in the used market.  Actually, the weakest link in my system now is the DAC, if I judge by the price.  I am using a Audio-gd NFB-3 which is an entry level DAC bought 8 yrs ago.  I originally plan to upgrade that first.  I bought a Topping dx7 pro, it is too bright for the X5 and the Bluetooth keeps dropping so I returned it.  Maybe it will match well with the Harbeth to bring some sparkles.  My Audio-gd now is a very smooth DAC.  
@arturgorniak is the extreme case of someone who believes 40 series Harbeths need serious power.
https://forum.audiogon.com/discussions/harbeth-40-2-40th-anniversary-with-simaudio-mono-blocks
At the other extreme, I've read posts from people driving them with tube amps well under 50 W.
But I think you are reporting the lowest.
For a real comparison/contrast, I think you need to aim for 100 or 150W SS.
@twoleftears I downloaded an app and finally got the number on how loud I listen to.  My max volume is around 90dB.  Average can get up to 85dB.  There is also a weighted average which I don’t really know what’s the difference, but it is much lower at 78dB.  I am at about 3.5-4 m away.  
I got a suggestion from someone that, I should leave about 20dB head room, ie. 100dB at 3.5m. Then I plug into a calculator in a web site, it turns out the following:

60W -> 94.95 dB
80W -> 96.2 dB
100W -> 97.17 dB
120W -> 97.97 dB
150W -> 98.93 dB

So, it seems your suggestion of 100-150W is spot on.  Then, it seems the only option is SS.  I am a tube guy, have switch to tube amps 8 years ago and never look back.  I am a bit hesitant to go back to SS gear.  So, my first decision need to be made is, do I get 60-80W tube amp, or 100-150W SS amp.

i read comments both ways.  Some say he try SS and don’t like it, then get a McIntosh 275 and very happy.  Some say he move to Luxman and Accuphase and they are as good as tube.