Pioneer's earlier class D amp, the SC-07 apparently had issues with 4 ohm speakers according to AH:
https://www.audioholics.com/av-receiver-reviews/pioneer-sc-07/sc-07-measurements-and-analysis
The newer ones such as the LX701 shouldn't have such issues any more, one would think.. They claimed:
https://www.pioneerelectronics.com/PUSA/Home/AV-Receivers/Elite+Receivers/SC-LX701
So I can think of a couple reasons why the LX701 may fall short of what you expected:
1. This newer crops of Pioneer class D pioneer still doesn't play well with the big RTi 12 towers (likely 4 ohms).
2. You ran MCACC, and likely didn't do it exactly the same way you did with the 94TXH.
3. Some expectation bias involved.
I suspect 3) just might play a little part because you seemed pre-conceived with the notion the 125 vs 135 vs 140 W could be an issue, when in fact those slight differences that are also typically specified and measured under different conditions, are insignificant and practically irrelevant to the audible difference you appear to be hearing.
1) and 2) are likely the real reasons, especially if 3) isn't.
The RTi12 are specified as 8 ohm nominal, but in reality they most likely are closer to 4 ohms as seen by an amplifier, based on the fact that it has the following driver array:
2 X 5-1/4 inch mids
3 X 7 inch woofers
With that many mid range and fairly big bass drivers, it's pretty hard to get the overall impedance to be as high as 8 ohm nominal. I am sure it can be done, but likely too difficult, therefore costly for Polkaudio to bother doing. @TLSGuy or
@Dennis Murphy may disagree, but I think there is a better chance that they would agree, that the impedance of the RTI12 are most likely lower (closer to 4 ohm) than the specified 8 ohm.
MCACC - If you want to compare sound quality between the two, you should use movies and music that you are very familiar with, make sure they are playing at the same spl (sound pressure level),and use pure direct mode.
If you use any other mode, the effects of MCACC will skew the results, that is simply a fact, that it will happen. Assuming your furniture in the room is exactly the same, you will still get significant difference results each time you run MCACC if the mic positions and the noise floor are exactly the same. So it could have just so happened that you MCACC run you made with the LX701 did not get the same (better I assume) results you got with the 94TXH.
Regardless, now that I know you have those towers, and it sounds like you do like high spl, I highly recommend you get a 200 WPC or higher (the higher the better) power amp. The remaining puzzle is, if you are not happy with the LX701, it is hard to explain why you seem fine with the 140 W 94TXH, that is still just a receiver, albeit a decently powerful one.