mono lp replay?? do you set up another TT for it?

I too have many mono LP's and enjoy them very much. I think a stereo cartridge is fine for modern mono pressings.

78 RPM mono records are another story. They are cut vertically and require a different cartridge to play properly. Benz Micro manufactures a mono 78 cartridge designed especially for the purpose.

If you decide you must have two cartridges and do not wish to give up your Linn, consider a tonearm with a detachable wand. Careful planning with cartridge mounting, spacing and weights allow that second cartridge and wand to "plug and play" with only volume adjustment.

I agree with Albert on both points. If you want to try it out, you could get a Lustre GST-801 tonearm w/detachable headshell, and get a Denon DL103 for stereo, and the Denon DL102 for mono. The DL102 is a mono-only version of the 103. There may be other carts out there for mono only, but the Denon is the one that I know about.
Personally, I find my stereo cart to do a fine job with the mono recordings.

Lyra makes a cartridge specifically for mono:

http://www.lyraaudio.com/HelikonMono.htm

I enjoy mono recordings with my stereo cartridge. In fact, I find some sound better, in many ways, than stereo. I read somewhere recently that jazz, in particular, is made for mono (or was it, mono is made for jazz?). My original mono LP of "Time Out" has a much more focused sound. The instruments have much more weight and tonal quality.