EVENTIDE! While word association might lead you to harmonizer it does reverbs well. It's certainly unique with the bandpass delays, harmonizers, etc. Since you just sold an MX400 that's reason 2, don't get something remotely similar to what you just ditched. Lexicon is lush and cool for pop records and loads of stuff, but I tell you, I'd love to have an Eventide!
As for clocks, you need one if there are more than two clocks in a digital system. Per your description it seems everything is feeding the RME and you could probably slave everything to that. To notice any sonic difference from a clock you'd have to get a really nice clock like the Antelope Audio ones. They can even slave to the Atomic Clock!
http://www.sweetwater.com/store/detail/OCX/
I have a BigBen and didn't notice much of a difference immediately. After it had been tirelessly working for months my 2yr old decided to tweak out and push all buttons associated with a light in my rig. Well, I'm working away on a current project and just kept thinking that there were scabs all over my audio, old crusty ones. What is wrong?! Well, the DM24 was back on it's own internal clock and it was not good. A good clock, in my experience, may not make an immediate difference, but you 'll know it when it's gone!