I guess this depends on why you want to use the Syn as a pre-amp to the Lyr+. Will you be using the DAC on the Syn at all? Are you wanting to send the shaping (i.e. width and presence) controls to your headphones through the Lyr+? But yes, you can stack by sending the Bifrost 2 RCA out to the RCA in on the Syn, the RCA out on main L/R channels from the Syn to the RCA in on the Lyr+, and then the Lyr+ RCA out to your speaker amplifier.
I would actually consider using the Lyr+ as your pre-amp, and treat the Syn as additional processing before your speaker amps. Same configuration that you have now, but send the Lyr+ RCA out to the Syn RCA in, and then the Syn RCA outs to your speakers amps. I would leave the Syn main volume pot at its maximum, and use the Lyr+ attenuator for volume control. You get the benefit of sending the tube-ness (or solid-state-ness) from the Lyr+ to your surround speakers, a better method for volume control, and the Syn stays out of the signal path to your headphones.
I saw the same review, and kind of questioned it at the time. I personally haven't noticed anything. But again, my system isn't exactly the most resolving system. I love how it sounds, but I'm not going to suggest I can hear the tiniest resolution of every detail. My source is the original Modi Multibit sent through a Saga S to the Syn. (Same as I recommended for you, I have Syn volume pot all the way up and use Saga S attenuator for volume control). For speakers, I have the original ELAC UB5 and Emotiva BasX A-100 amp. My headphones are Hifiman HE400i and BeyerDynamic DT880. When I sit back and listen to 2 channel music (i.e. process and shaping turned off on the Syn), it still sounds great, and I assume my headphones sound as good as they can considering the quality of headphone amp in the Syn.
Your results may vary depending on your equipment. But also, Jason Stoddard has said in his live streams, that the Syn is clean enough to send a Yiggy through it, and not get it stepped on. However, if its something your worried about, taking the Syn out of the signal path to your headphones would solve that issue.