Vandersteen 2wq x-over help

The M5-HP sounds better than the X-2 if your willing to spend $500 used -$900 + new for a pair (do you have two subs?) I may be wrong but I think it's a personal room related preference for what works/sounds best in your system/room. My guess 75Hz or 100Hz but it really is what you prefer.

Does anyone use the wx2 as a permanent solution?

No, its absolutly Half Ass.
As Stated in the manual
Its a temporary assessment box to experience the optimum choice setting for your particular amp, and Vandersteen 2WQ.
Adjust for the best transistion acording to the input impedance of you amp.
Once you have experienced the best setting you simply contact your Vandersteen dealer for the premium grade X2
Fixed unit.
RCA at $120 the pair
XLR at 225 the pair
For even better performance
Variable Battery bias type High Pass units called
Vandersteen X5 RCA and X5 XLRs
are available at 895 and 995 the pair
Cheers JohnnyR

The X5 XLRs are highly recommended.