I want to make a playback loop selection then let it play while browsing through wav loops that will play at the project tempo - in time. You can do this in Reason with REX files, and also in Ableton Live with any media. 
 
I've been previewing loops in the browser at project tempo, without my project playing. If I stumble across one that I feel might work, I drag it into the timeline and then play my project. 
 
I'm thinking about running Ableton Live as a ReWire client, or just switching over to Ableton Live altogether. 
 
I've also searched for a VST that could help. I tried NI Maschine 2, but that didn't work the way I hoped.

You can do what you want in SONAR with any media (although properly edited REX and Wave files will sound better). Do you have the following checked in the browser's media drop-down menu? 

• Preview at host tempo
• Auto-preview
• Loop preview 

Also note that SONAR's browser is a multitrack loop browser if you hold Ctrl and click on multiple loops. This is great for construction kit loop libraries, and the process is described in Week 7 of my Friday's Tip of the Week thread. The loops have to be in the same folder, though.
 
And don't overlook what else SONAR can do with REX files.

Well then. Lol. Thanks. It does work when you check all of those items off. 

Check out the multitrack loop browser aspect. It's quite cool.
 
Note that SONAR is the only DAW other than Sony Acid Pro which can edit and create Acidized files. I also wrote about a way to create "faux" REX files if you don't have ReCycle.