After reading some reviews it sounds like the ML Motion series may be geared to be more of a "front row" perspective rather than further back, so meaningfully increasing depth of stage may not be in the cards. Apparently pulling them out further into the room didn’t help too much, which would make sense if they’re designed more for an upfront presentation.
Since this is a system being used for both multichannel and stereo I’d shy away from tube amps. No sense burning up tubes to watch TV or movies IMO. Although your speakers are stated to be 90dB efficient they’re also rated at a nominal 4 Ohms impedance, which means they probably dip below that somewhere in the frequency range (the Motion 40s drop below 3 Ohms at around 100Hz so likely the 20s dip around there too). Anyway, for all these reasons I’d avoid tube amps. I’d also recommend a good stereo amp rather than monoblocks -- no strong reason to pay up for two boxes in this situation, again IMO.
Just FYI, adding a stereo preamp to your system wouldn’t add much complexity at all -- I know that sounds a little hard to believe, but it’s true especially if you get a preamp or integrated with a HT bypass. My best and most cost-effective advice, and what I’d absolutely do myself if I was in your situation, would be to add a good solid state integrated amp. You’re only adding one box to your system, and you’re killing two VERY important birds with one stone by adding a better dedicated stereo preamp and amp. Going direct from your Marantz doesn’t change the fact that the signal is still passing through the preamp section (and the rest of the electronic morass) of the prepro, and at that point the damage is done. Something like a Parasound Halo Integrated or a Hegel H190 (among others) would likely bring very significant improvements to your 2-channel experience. Here’s a link to an H190 review that actually uses your speakers in the review system: https://www.soundstagesimplifi.com/index.php/equipment-reviews/29-hegel-music-systems-h190-dac-integ...
Incidentally there’s an almost new H190 available here now at a $1000 discount. Best of luck in whatever you end up doing.
Since this is a system being used for both multichannel and stereo I’d shy away from tube amps. No sense burning up tubes to watch TV or movies IMO. Although your speakers are stated to be 90dB efficient they’re also rated at a nominal 4 Ohms impedance, which means they probably dip below that somewhere in the frequency range (the Motion 40s drop below 3 Ohms at around 100Hz so likely the 20s dip around there too). Anyway, for all these reasons I’d avoid tube amps. I’d also recommend a good stereo amp rather than monoblocks -- no strong reason to pay up for two boxes in this situation, again IMO.
Just FYI, adding a stereo preamp to your system wouldn’t add much complexity at all -- I know that sounds a little hard to believe, but it’s true especially if you get a preamp or integrated with a HT bypass. My best and most cost-effective advice, and what I’d absolutely do myself if I was in your situation, would be to add a good solid state integrated amp. You’re only adding one box to your system, and you’re killing two VERY important birds with one stone by adding a better dedicated stereo preamp and amp. Going direct from your Marantz doesn’t change the fact that the signal is still passing through the preamp section (and the rest of the electronic morass) of the prepro, and at that point the damage is done. Something like a Parasound Halo Integrated or a Hegel H190 (among others) would likely bring very significant improvements to your 2-channel experience. Here’s a link to an H190 review that actually uses your speakers in the review system: https://www.soundstagesimplifi.com/index.php/equipment-reviews/29-hegel-music-systems-h190-dac-integ...
Incidentally there’s an almost new H190 available here now at a $1000 discount. Best of luck in whatever you end up doing.