Not Leon, but have plenty of experience working with Synergy (and other tube stuff) in combination with modelers (including an FM3) and...
...there are reasons to want to use a tube pre-amp along with a modeler, but "tonal improvement", in my experience, isn't really one of them. Knobs and hardware and avoiding the rabbit hole of swapping to different amp models frequently, etc., are all benefits.
I also think running a tube preamp in the effects loop kinda misses one of the biggest benefits of running a tube preamp in conjunction with a modeler: having your guitar (or pedalboard) hit an all-analog front end that you can then gain-stage appropriately for the digital world. You can use dirt pedals, including ones with massive output, and not worry about whether or not your clipping the converters, etc., Running a tube preamp in the loop of a modeler just adds one more layer of back-of-the-mind "wait, is the digital stuff, including an unavoidable round of conversion even if I'm not using any pre-effects, depriving me of all the analog goodness?!?!?!" worry that just makes the whole expereince less fun -- and for me pretty much the entire point of running analog preamps with a modeler is...fun.