Which to keep! Pick one of these two soeakers

While ultimately it depends on how they sound to you. However, it also depends on your electronics and how much time you have spent with the Martin Logan. 
 

It would be really helpful to know your components and venue… there is a place under your UserID to create your virtual system. It is easy to attach photos and ID equipment there. This would help us better determine how to help.

Just in general, I suspect the Martin Logan’s are more challenging to power and position and perhaps you are not getting the optimal sound from them. They can sound very good.

The previous owner of the Martin Logans had them bi-amped with an Audio Research VT100 (feeding the subs) and ADCOM GFA-555 (feeding the panels). The Audio Research SP16 preamp feeds both amps. I have all of this gear now but I’m only running the VT100 biwired to both the panels and subs in parallel. The room is carpeted and about 12x20 or so. I’ve beed auditioning the Spendor and ML with the Audio Research pre and tube power amp.

I'm all for using one amp for both panels and subs. I've a hard time thinking the Adcom would be a good match (soundwise) for the panels. When you set these up, I assume in a somewhat triangular configuration, you need not be as concerned with sidewall reflections (as you would with boxes, but the rear wave is a potential problem. To hear this speaker best it should be about 6 ft off the wall behind it. Dispersive material on the wall wouldn't hurt much either. Good place to start anyway. Take your time with setting up these speakers before you come to any final conclusion. 

IMO you should be able to pick your own speakers? 

I guess I don't understand what you are asking? You own both pair now, yes?

You can configure them how you like, yes? You can listen to both and decide which pair sounds best? no???

How many hours do the "new" panels have on them.  They need about 200 hours to break-in.

I'm a panel guy and biased, so I say keep the Martin Logans...