" Also, wouldn't the 5350 be a "waste" if used only as a power amp?"
Not at all. That's how you are using it now. Unless you bought the active line stage add on card, the Creek has no preamp. Just to be clear, I'm not bashing the Creek in any way. I've gone through an enormous amount of equipment in my life, and of all the components that I wish I never sold, my 5350SE is number 1 on my list. That said, it does have its limitations.
" @mb1audio02 I would be interested to understand your considerations when you recommend a preamp?"
Aside from the fact that you don't have one, reading through your description on how your system sounds, and where you would like it to go, the preamp seems like the most logical place to start. I need to stress, however, there's usually more than one way to fix a problem in audio. Throwing a new pair of speakers, or a bigger amp at the problem in a situation like yours, is the most common fix. Its direct and easy. The problem with that is it's a band aid type of fix. When it comes to timing, pacing, spacial placement, dynamics, dynamic contrast, etc... the preamp is a fundamental component. If your system is missing a fundamental, everything you put on top of it will fail at some level. I know your Creek very well, and I know MA very well. If you think that combo sounds too laid back, then a preamp is the right fix. If you choose to fix the problem a different way, with new speakers for example, you may have a short term fix that sounds acceptable, but the minute you upgrade to an integrated amp with an active line stage, or just a stand alone preamp, you'll almost certainly find your system to bright. And that's when the real fun begins. At this point, most people think they like all their components, and they're properly matched, so they start playing with cables, tubes and any other tweak they can get they're hands on. But all they're really doing is wasting money.
(I typed all this out really fast because I'm in a hurry, so if I messed up explaining something, just post and I'll try and fix it.)