Which to keep! Pick one of these two soeakers

I agree with those suggesting optimization can be a big factor  You've had one for decades and they tend not to be too particular on set up  ESL's typically a different story   I would guess the Martin Logans have greater potential for realism, accuracy, esp for seated listening in the sweet spot  

Your opinions are super helpful, there are little bits of information mixed in to each that helps me piece together some of the greater experience you all have with these types of speakers. To those who are a little rubbed by me asking for the experience and opinion of others, I find it extremely helpful, and it helps me to bring attention to things I may not have otherwise consciously been able to consider in my decision. It also adds confirmation to the experience I've had, or helps me reconsider things that maybe I got wrong. There have been many useful points brought up here. 

I mainly bought this guys system for the VPI turntable, Audio Research pre-amp, CD Player and VT100 MKIII power amp. Those three items are nicer than the core of my #2 listening room that is also my home studio. There was a whole bunch of other stuff in the rig like a transient noise reducer, the Adcom amp, the speakers. I was initially thinking I'd sell the speakers but they turned out to be pretty nice which is what got me into the predicament of considering my Spendors for replacement. I wonder if I could fit both sets of speakers and wire them to an AB switch. They are different Ohms though, The ML are 4 ohms and the Spendors 8 ohms. I wonder if there is a way to wire them up to a couple of switches and turn off one, turning the other on. 

There was a Marchmand EQ and a Marchmand tube crossover also hooked up to this system that got sold out from under me. I would have really liked to have those to play with on this setup. That may have helped a lot with the Martin Logans. I feel they need a little high rolled off at times from what I'm hearing, maybe the lows bumped just a tad. Same with my Spendors, they are really bright but I really like the clarity 99.9% of the time. I have a sub that I use to fatten the room up. 

Is there a good EQ that I could invest in that ya'll would recommend? I really liked the Marchmand controls, it was 5 knobs and pretty simple and seemed well built.

i think the spendor s100 are of 1990 vintage, so not so old. since you have a sub i'd pick them over the martin logan, which look cool but didn't sound that great to me.

Whatever you do, keep the S100s because you won't realize just how good they were until they're gone. Many of us would give a limb for those. MLs might impress short term but will simply fall short in the overall musical satisfaction category. Btw, these are a speaker that both sound great and measure just as well. But if you sell them, I'll buy them!

@loomisjohnson my spendors are below 1000 serial number and are probably pretty old. I'm not sure how many they made in the 70s/80s but it would not suprise me if they are pre-1990. I wonder if I rebuild the crossovers if they might sound a lot better on the lower register. It's possible that the crossovers are out of spec and not pushing enough low frequency down to the subs.