I also ask this because I have a set of Joseph Audio speakers too that the LM doesn't push hard enough.
I feel your pain big time brutha! Big JA fan and I too couldn’t stand watching those pricey tubes burn away while watching Two and a Half Men reruns (which JA speaks do you have BTW?). But I also know your LM amp is pretty amazing, especially for the price. I’d do the tube pre and sadly sell the LM unless you don’t mind switching it in and out for 2-channel listening (something I’d seriously consider doing if I was you, but I’m kinda nutty that way). There are some tube preamps that don’t power the tubes when in HT mode (Rogue, VAC, Backert maybe?), but they’re few and far between and who knows if those few would be your cuppa. But preamp tubes don’t tend to be as pricey and last a lot longer so maybe that’s not even an issue for you (It’d still grind my gears knowing I’m burning tubes to watch movies/TV, but I have issues that way). I’ve now even confused myself and probably helped you even less. Use the Cherry, get the CJ pre you know you want, and enjoy the crap outta the JA speakers as a bonus. Preamp tubes be damned. Life’s too short. Go for it!