Agree with Ricardojoa's comment on the need for a sub with certain types of music.
Also, It will be defined by the levels of bass YOU personally are satisfied with!!!!
The SongTowers excel with most types of music. On some occasions, a bit of an assist from a sub made everything much more enjoyable, with certain music types, for my ears.
I was happy to have the subs add the extra bit they did with the SongTowers. HOWEVER, I do not need the subs with my Gemme Tanto V2s and I do not use them in support of the Gemmes, even with ht. Just bringing this up since it has not been mentioned in the other posts.
Getting back to the STs. If you will be dipping into any Reggae, Hip-hop, Electronica, modern Spanish, and any modern/alt rock, etc. a well integrated sub will make you appreciate the SongTowers all that much more.
I found integration with two different subs fairly easy. The subs stayed OFF for about 60% of my 2ch listening.
Since you will be doing ht, sub duties, when called for with 2ch music will be a terrific bonus.
So its 45 less how many days now????