HELP Isolating a problem - preamp or power amp

Hi All,
I have a problem with my audio system that I need help with. The system is a CJ ACT2 preamp driving a CJ MF2550SE power amp.
Yesterday my system stopped working. By this I mean that the volume out of MF2550 is not very high. ( even at the setting of "45" on the ACT2 (which used be optimum for my room) its low. The problem is independent of the s source.
Here is what I have done so far.
- Checked the ACT2 outputs (MAIN 1 and MAIN 2) using a pair of headphones and they seem to be working.
- Disconnected the MF2550SE and connected an integrated amp (Prima Luna) to the ACT2 
and the system worked. My concern is that the preamp of the Prima Luna might be masking a problem with the ACT2.
I was wondering if I can drive the MF2550SE directly with a source (maybe a CD player) and if that works it would indicate that the preamp is the problem.
Suggestions much appreciated