You say you haven't decided on speakers yet, but are leaning towards the Ushers. IMHO, I'd look for speakers that are easier to drive, and stick with the Forte 4a. It's in a totally different weight class sonically than the Parasound A21. IMHO, you'd have to spend about $2K on the used market to equal the Forte 4a sonically and aquire more power, it's a real giant killer.
Cheers,
John
Cheers,
John