Threshold SA/3 with MF Pre 24 CD and Classe DR-5

I have a simple system that I'm trying to get right and hope you can help. Started with a Levinson 27 and Classe DR-5 with a Meridian 207 Pro 2 box unit. Speakers are IMF TLS 80 II's on factory stands. The Meridian died and I thought the Musical Fidelity Pre 24 looked good as I do some tape mixing - it got great reviews (uh huh) too. I've now changed the 27 (with which the Pre24 sounded good) to a Threshold SA/3 that I much prefer in my system but (it's a big but) it sounds thin and quite shrill and edgy (no complicated words ok) when coupled with the Pre24; however with the DR-5 the SA/3 sounds just about right for me. I'm happy to use the Pre24 as a cd player source into the DR-5 and on into the SA/3. The outfit doesn't seem to be very cable sensitive at all. Question though: any ideas why the Pre24 used as a preamp into the SA/3 would sound so bad - but I'd like to stress again that's only in in my system. OP inpedence of the Pre24 is 50 Ohms - does the SA/3 need different? I've read that the DR-5 works very well with the SA/3 and can certainly confirm that - very synergistic.