Should I replace my Marantz SR880 MkII receiver?

I've heard the Rotel pre/pros and they are hard to beat for the money. I'm not aware of any receiver that has a preamp stage that's anything beyond pedestrian. In any case, I'm fairly sure the Marantz isn't doing justice to that Rotel amp. If it were my choice, I'd get an inexpensive pre/pro. You may find that things "open up" if you do. The Rotels are quite good; also the Outlaw (950 I think?) would be a low cost alternative. Either brand will give great value for the money.

Also, be aware that you get killed in the used market on receivers regardless of brand. You'll have to make that choice should you go with a pre/pro. Or better yet, relegate the receiver to a second system where you do your less critical listening/movie viewing.

For what you're 880 is worth, you won't get a better pre-amp out there. The weakness of the receiver is the amplification. Stick with what you have! I will say that I've had both a SR880 as well as an AV550 and just upgraded to an Anthem and notice a huge difference (but look at the price difference)
Happy Listening.