Subwoofer advice

Sorry in advance for the long post but I thought it would be helpful to give as much context as possible in order to receive the most accurate advice.

I am new to this subwoofer thing. Currently I have a traditional stereo system. Dedicated amp/pre/dac and Harbeth 30.2 speakers. Now I am not unhappy with the bass output of the Harbeths, in fact, I was impressed with the useful output I heard under 50hz, even though they're only rated to 50hz.

I was thinking adding a powered sub or perhaps two subs would be beneficial as my Harbeths start to roll off around 40hz and I get no useful output below 30hz or so. I don't consider myself a bass head but I do like the occasional Daft Punk song, and know that a good sized woofer can make all the difference in a system. I think the Harbeth Radial driver is swell, but it struggles at higher spl, under 50hz. (Herb Reichart mentioned the logo will rattle against the enclosure with strong subbass, he wasn't joking)

I don't wish to add a processor into my chain for subwoofer management, even though I could (my preamp has home theatre bypass) I would rather do the "traditional" way where I set the crossover of the sub to maybe around 40hz and adjust placement in room manually by ear.

My room is small, 10x14.

My questions are thus: what brand should I look to get?

Should I get one or two subs? One to start?

What considerations must I factor in for seamless integration?