Looking to Add to my 7100

Here ya go!

MONOPHONIC (BRIDGED) OPERATION A rear panel MODE switch allows the MC7100 to be used as a normal stereo amplifier, or as a bridged mono amplifier. Connect a shielded cable from a mono signal source to the R (right) /MONO input jack on the MC7100. Set the mode switch to MONO. Only the right channel LEVEL control func- tions in MONO operation. The outputs must be connected as indicated in HOW TO CONNECT OUTPUTS for proper mono operation.

The owners manual!
https://www.manualslib.com/manual/1018221/Mcintosh-Mc7100.html

mantaman. You might not like what I am about to tell you, but I feel my information will be critical to you. First, on a positive note, your Solo speakers are / were an excellent design for their size, and a wonderful 1st product line from Phil Jones, the primary designer behind the Platinum line. Now, the not so positive news. To achieve the excellent low frequency response from such a small cabinet and small woofers ( a pair of 4 inch, per cabinet, if I recall ), the speakers are very inefficient, which you are experiencing with your single 7100 ( I am familiar with it ). In fact, the Solos are some of the most inefficient, high quality speakers, I have come across ( and that, is many ), in my journeys ( I sold, and installed, the Platinum line, back in the day ). Besides them requiring a lot of power from an amplifier, the Solos also dip in impedance, at frequencies where much music is contained, to around the 3 - 4 ohm area. Because of this, I am not recommending a pair of Mac 7100s, bridged, as I am recommending another power amplifier altogether. Something with 200 wpc, and high current delivery into lower impedance loads. As your gear is pretty old, I am thinking a budget might be important to you, however, there are a number of amplifiers, new, and used, that are reasonably priced today, that will get the job done ( I used Krell and Levinson ). Other popular product lines, such as McCormack, Parasound, Classe, or even a higher powered Mcintosh, would fit the bill ( there are quite a number of others ). I am not expecting you to understand this, and, I am sorry for not answering your question, although, your question has been answered by another, above. However, others may have different experiences, and opinions, than mine. YMMV. Enjoy your journey. MrD.

Page 12 shows how to connect. Put the rear switch on mono. Connect the positive side of the speaker to right positive speaker connection of amp. Connect the negative side of speaker connection to left positive speaker connection of amp. Run a pair of RCA cables from preamp to the right input of each mono amp. You now have two mono amps! BTW, to be safe the speakers should be 8 ohm!

Mrdecibel-  Good point. I would have mentioned it, but since the OP was clear he only wanted an answer to his specific question, I figured let him discover for himself.  Perhaps he will be more inclined to listen to your sage advice.