Those are not bad at all with built in x overs glas cones and fabric tweets. I've heard the 6" comps and coax but not the 6x8. They sounded very good with hu eq dialed in.
If it were me I'd run them front and back. It's easier to tune with same models front and back. Pioneer speakers take a beating and sound good from my experience. There's nothing wrong with them at all and you already own them.
I always yank 6x8 5x7 speakers and make 6.5 wooden baffles adapters because there is way more 6.5 choices than 6x8 5x7.