Bottom line is, if they don't sound very good on what you created them on, odds are they won't sound THAT much better on anything else. 90% of it still comes from the PERFORMANCE... Does it have dynamics, does it have impact, does it sound brilliant already? Is the playing stellar? An undynamic, uninspired part through the best samples in the world sounds no better.

And most modern MOTL and upwards arrangers are capable of sounding quite fabulous in the right hands. I would try to make sure that what you already have sequenced knocks your socks off first, before I went in a studio and spent hard earned cash...

Sampled loops are OK, but they suffer from seldom having Intros, Endings, even Fills that are appropriate to your song. And they have to be matched VERY carefully to the groove of the SMF (or vice versa) or they sound tacked on. There is no 'magic bullet' that will turn something you are dissatisfied with into something amazingly better. It's a long and slow process to integrate a variety of 'replacement' samples into something cohesive. And if the part in the first place sucks, it is pretty much impossible...