1 preamp out to a 2nd preamp in any harm?

The mono signal will be generated via y output from the preamp going to the monoblock.

That will probably work ok, but if the preamp has particularly low output impedance sonic issues might result.

The monoblock will connect via speaker wires to the crossover system in the rythmic plate amp with one feed going to the sub and one feed going to the woofers.

Again, providing a mono signal to woofers that have to handle frequencies into the mid-bass region (or even higher) will not sound good, assuming the overall setup is stereo.

The Sub has a rythmic plate amp that produces a hum in the system when connected via RCA, the hum disappears when connected via speaker inputs.

You could most likely eliminate the hum in the RCA configuration by inserting a Jensen Iso-Max transformer into that path. Consult with them by phone for specific model recommendations.

http://www.jensen-transformers.com/an/an004.pdf

http://www.jensen-transformers.com/index.html

Again I have know idea what this will sound like I just do not want to destroy anything finding out.

As some of the others have said, you will not hurt anything, as long as you don't have both volume controls simultaneously turned up to unreasonable levels.

Regards,
-- Al

Luked, get an Art Pro Audio CX-311 electronic crossover ($109 online), and feed it from your preamp via Y adapters. The CX-311 has a dedicated subwoofer line output with adjustable frequency and level pots. I own two CX-311s.

Elizabeth you were spot on and this does sound great. I only went from second preamp to sub to rear speakers making them mono and it did everything I needed. I did not mess with crossovers in each speaker to obtain the complete seperation of woofers, to much work at this time and I like the results of what I did. I can fine no deterioration in sound and the rears set at a lower volume then the front actually help with the sound stage versus the two sets in stereo which in my large room and old ears had a poor front to back seperation. Thanks for the encouragement. Bass is now extremely tight a sound I have always preferred.