Amp setup question!

I have got an integrated with a pre out and a pair of monoblocs.I use a front pair and a sub (active), and I used to power the fronts from the integrated speaker oputputs and the sub from the preout.I've now got these lovely monoblocs I want to use on the fronts, via the pre out. So can I then the use the vacant speaker outputs on the integrated to power the sub (1 pair only) ?
Will it work? What about differences in speaker impedance? Will it matter that I don't have one pair of speaker outputs connected?
The answer would probably be to use a multichannel preamp, (not a processor) ;does anyone know who makes them? I use a universal player of high quality for all my music, CD, SACD, DVDA etc. which does all its own decoding, hence no need of a processor.
The integrated is great quality, though and I don't really want to get rid of it (AVI Lab series).
All responses gratefully recieved!