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

Yes of course you can. Just be aware if the CDP has variable output to turn it down. Or it might be really loud. But even if you don’t have that you can still test, just use a track that starts at a low volume, just in case.

Generally speaking the output from line level sources like a CDP is more than adequate to drive most amps to full power. Most preamps in other words don’t amplify they attenuate. This is why people are able to run these things direct, they have plenty of output. So be aware and if you hook it up direct and still can’t get good volume its definitely not the CDP. 

In this case, from the sounds of it, its the amp.
Thanks a lot milercarbon6. Much appreciated.

Just a follow up. I connected my Linn Akurate DS directly to the MF2550. Tt low volumes it was OK but distortion , cracking and popping came in as the volume want up. I guess its clipping?

No, clipping would be real loud but no way should there be cracking and popping. Now it really sounds like its the amp.