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

I think what you want is very common and one solution is to use Y connectors on a single preamp's main outs with one leg of each going to 2 different amps. That would be less likely to distort your distortion in chaining preamps.

I don't understand several things about this:

1)The woofers in the main speakers will presumably be handling much higher frequencies than a sub would normally handle. Assuming that the overall setup is intended to provide stereo (is it?), I don't see how it would make sense to provide a mono signal to those woofers. Driving a sub with a mono signal can be appropriate only because the frequencies it is reproducing are very low.

2)How are you generating the mono signal?

3)Assuming the sub is a powered sub (is it?), containing its own amplifier and providing a level adjustment, I think you are saying that the reason for including the second preamp is to allow level-matching between the woofers in the main speakers and the mid/hi drivers of the main speakers. Am I understanding that correctly?

4)Can you indicate the make and model number of the sub, which may help to clarify some other things I am uncertain about?

Regards,
-- Al

There are a couple of unique problems with the current set up. 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. The mono signal will be generated via y output from the preamp going to the monoblock. 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. If this produces distortion or an echo effect I may just run a mono feed to the back speakers from the sub. Again I have know idea what this will sound like I just do not want to destroy anything finding out.

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