I'm going to cut across the grain here. While I agree with the theory that lightening the burden on the main speakers can reduce amplifier and driver loads (See John Potis Bryston XO review), the Druids are really efficient and don't have the same ground to make up as other speakers. Further, their drivers are wickedly burly and don't exhibit the strain with bass heavy stuff that other speaker/amp pairings do.
Further, the Druids' bass quality is excellent. I don't see the utility in eliminating it.
Bear in mind, whatever sub you get will have its own low-pass XO. If you do go the XO route, you really only need a high-pass filter for the mains. When I had my Definition Pros I used these, high pass at 70 hz:
http://www.hlabs.com/technical/crossovers/These won't mess up your sound and run about $30 a pair. I'd start here if you want to dip your toe in the waters.
When I had the TacT units I played with all manner of XO settings with the Pros, I determined a steep setting at 65 hz was best. Whenever there was overlap between the mains and subs, a muddiness would set in that would disappear once the overlap was removed. I thought it was a phase issue but now that I have the Def. 2s with much better bass bin tuning, I can run the subs to 125 hz where the mains go down to 40 hz and they blend very well.
In fact, this creates a bipole at those frequencies in my room which adds space and depth to the aural illusion.
With the TacT units, I was running a $4k DAC that fed straight off the native TacT frequency. All told, it was $10k of processor. I came to a point where I believed the processing itself was holding me back and I abandoned it. I would be wary of Digital XOs though I know they are gaining popularity.
For analog XOs, my sense is dealing with it at line-level is better than high/speaker level but if you can avoid it altogether I think that's best. I don't think there's such a thing as a transparent circuit, just ones that are less bad. IMO, of course.
If you want to get a sense of what's actually going on in your room, a quick sweep with the Room EQ Wizard is invaluable. If you want to truly integrate a sub I would think it or something similar would be essential.
At the end of the day, I would try hard to get it to work without additional stuff. You know for a fact adding components won't help transparency, at best they'll do nothing but split signal. Spending money to add complexity should be a last resort if you absolutely need additional functions.
If you DO decide to go full-bore XO, the ability to run a parametric EQ off the bass output would be THE reason to do it. If you go that way, don't half-ass it.
IMO, grain of salt, disclaimer, YMMV
