since you are using MS, how have you config your system. Large/small, bi-wired etc.
All my MS speakers are set to small even the floorstanders. I have the satellites crossed at 100Hz and the floorstanders at either 60 or 80Hz. The rest goes to the sub. None of the speakers are bi-wired.
What I mean by difficult to drive is MS tend to have a low sensitivity so it takes some good juice to get them singing. The LPA-1 pushes 125wpc into 8 ohms and 225 wpc into 4 ohms which is plenty for my setup.
Mordaunts are a tad bright, but Axiom, Klipsch, BA, and some others are clearly brighter by comparison. I like Mordaunts because they are detailed without being too bright or harsh. They definitely aren't warm speakers, but I find warmer speakers sound muffled to my ear and the Mordaunts are just right. It comes down to personal preference.
You have the MS908's which should put out good bass. It sounds like they might be underpowered. I'm not familiar with that Yammy receiver so hopefully someone has some insight.
You have some good equipment already so maybe swap out a few things and change some settings and all should be good. Or maybe it's time to audition some new speakers. Like mtrycrafts asked, what do you think is lacking in your system?