connecting mono amps to pre amp

The switch on the rear of the 2120 that selects between stereo and mono should be set to mono, as has been said.

The mode selector switch on the front panel of the 6100 should be set to stereo, assuming you are listening to a stereo recording. Setting it to mono would result in both speakers putting out the same sound, corresponding to the sum of the two channels of the recording. Assuming you are playing a stereo recording, what you want is for the left speaker to play the left channel of the recording and the right speaker to play the right channel of the recording. No damage would result, though, regardless of which of those positions the switch is set to, and regardless of whether a stereo or a mono recording is being played. The result would just be that the sound and the imaging would not be right, if you were to play a stereo recording in mono mode.

If you are playing a mono recording though, it would be preferable to set the mode selector switch on the front panel of the 6100 to mono, although it might not make much difference if you leave it in stereo.

Regards,
-- Al

Not to confuse you, but since you already own both amps, you may want to try a vertical biamp. (I'm assuming your speakers have 2 pairs of binding posts). For the most part, people feel that bridging a stereo amp degrades sound quality. How much will vary from system to system. A vertical biamp will allow you to use both amps unbridged and avoid any power differences that usually come up with horizontal biamping.

If you want more info, have a look at this.

http://vandersteen.com/audio-perfectionist-journal

Personally, I think APJ is the best reference on high end audio ever written. There's an incredable amount of useful information in it.  

That’s a good suggestion, Mb1, assuming as you say that the speakers can be biamped. To clarify, however, when the OP’s amp is used in mono mode it is not bridged. The two channels are paralleled, as I mentioned, via their output autoformers.  That is of course different than bridging. And depending on the specific design the result could very conceivably be no sonic downside, in contrast to the downside that is often attributed to bridging.

Regards,
-- Al

yes, I actually have two tube amps that are paired up with a mac c 26 that are bi-amping a set of paradigms. sounds really good.  Just looking at some options with the ma 6100 and the two 2120's. as mentioned in my user name I have some other pieces, sound craftsmen, nikko, marantz, kenwood. and a couple of pioneer spec pieces, still looking for a couple of other spec pieces to finish up that rack of stuff,  My wife thinks I'm NUTS!!!  buat as I told her I'm home more often since the Harley, and vette are gone. anyway, i really enjot the 2 channel stuff! 

i have a question regarding the 6100 mode switch, it says Mono (L+R) as opposed to a true mono that would only play one channel (the same channel) thru both correct??