Inexpensive pre-amp with mono switch & remote?

Hi, a few years back, I wired my house for mono with speaker wire. I would like to start using that wiring. To do it, I need to have a pre-amp with a mono switch, one that will "mix" my stereo sources to mono.

It needs a remote because I will be using it from another room (I was thinking to install a remote repeater).

The amp will be a Rotel 135W stereo amp bridged to mono. I will use a resistive network to allow driving multiple speakers. Haven't picked speakers yet, but they will not be expensive (perhaps $500 per pair, $250 each), and will be bookshelf-type.

I would like to keep the cost down as this is not my main sound system, and I am not expecting to buy real expensive speakers. Also speaker positioning and wiring are less than ideal.

I thought an old receiver might work, but come to realize that the "mono" button (at least on the one or two that I looked at) only turns the FM into mono. It doesn't turn other stereo sources into mono.

I demoed an Adcom 750 preamp about 6 months ago (for my main system). It has a mono switch that does what I want, but is out of my price range for this system.

Used would be OK.

I suppose there is probably some way to mix the output of a stereo pre-amp on the way to the mono amp, but this seems likely to degrade quality more than a mono switch on a preamp (but then, what do I know)?

Thanks for any help,

Eric