I am driving my SHL5+ with a 60 watt Ayre AX-7e right now. I had a similar issue to yours with my previous DAC (Modwright Logitech Transporter) going into the Ayre. I changed DAC's to an Ayre Codex and now my amp drives the Harbeth's louder than I ever play my music. My room is an open space that's 35 x 13 with an open loft above.
I have heard nothing but good things about your amp and I have owned a Saturn in the past so I am surprised to hear of your issue. On paper it should work fine. One thought might be to try a different speaker cable. I also use Castle Rock in my system and have no problems with it but my dealer tells me that he has had issues with Castle Rock with some British Electronics and usually uses Proac Speaker cables with his Rega gear. From own personal experience I tried a Rega Osiris with the Harbeth's and I did not like the match. However, that is just my opinion and YMMV.
donvito had a good suggestion about trying your Harbeth's in your other system.
Good Luck solving your issue!
I have heard nothing but good things about your amp and I have owned a Saturn in the past so I am surprised to hear of your issue. On paper it should work fine. One thought might be to try a different speaker cable. I also use Castle Rock in my system and have no problems with it but my dealer tells me that he has had issues with Castle Rock with some British Electronics and usually uses Proac Speaker cables with his Rega gear. From own personal experience I tried a Rega Osiris with the Harbeth's and I did not like the match. However, that is just my opinion and YMMV.
donvito had a good suggestion about trying your Harbeth's in your other system.
Good Luck solving your issue!