Since you mentioned crossover upgrades, I'll throw in my 2 cents.
I have owned 2 pair of F208's, the first pair were new, but were returned after auditioning. They did some things well, but I didn't feel they had the right price/performance ratio.
They were definitely not "tube friendly". I measured the same two impedance dips (3 ohm) that Stereophile did.
I later bought a second, "previously owned" pair (much cheaper) to explore them further.
Despite what Revel might claim, they use "off the shelf" SB Acoustics drivers. I think I came up with about $315 retail if you bought them yourself. Markup is expected, and they are nice drivers in other implementations.
What really hurts their performance, is the crossover. While well designed, the drivers integrate well, with decent imaging. The actual crossovers don't have more than $5 worth of parts in them.
This, to my ear, left a gray, washed out, lifeless sound. I heard this before taking them apart, so I wasn't "pre-prejudiced" by what might be inside. I have heard (and used) those same drivers elsewhere, so I know they can sound much better.
When I resold the second set, the buyer asked to hear the speakers I was keeping (Tyler H3's with modified crossovers).....I almost thought I was going to lose the sale. There was a huge, audible difference, throughout the range. And that was before I replaced the H3's crossovers completely, with a better design.
Anyway, as they say, ymmv. But it still irks me that Revel lists the 208's as "8 ohms nominal", when the impedance rarely rises above 6 ohms 20-20k. The the midrange and tweeter are both 4 ohm units, with 2, 8 ohm woofers in parallel.
As stated earlier, just my 2 cents, but there are quite a few superior cost/performance/sound speakers for the price.
Not trying to dissuade, just share what I learned. The 208's do look nice, and don't sound bad, they could sound so much better with just a few $ in parts.