Vandersteen high-pass filter with other Subs

I would like to relieve the midrange congestion of my Vandersteens by inserting a sub(s) with high-pass filter, per Vandersteen's own design.

I already own REL Stadium III subs and would like to keep them. Would there be any problem inserting a vandersteen x-over between my pre and amp (say, e.g. the one used in the model 5) and sending the outputs of my amp to the RELS, rather than to Vandersteen 2WQs? Or will I have some sort of impedance or other circuitry issue, of which I am currently happily ignorant?
That won't work as far as I know Judit. If I am not mistaken the REL uses a line level signal. The much higher voltage amp output the Vandersteen uses would overload the REL's input.

You mentioned midrange congestion before. Do you find this to be the case without the woofers? I still think it is your room sound, or a set up/placement issue. I would try moving things around first. Room tuning devices may be in order if all else fails. What are the rest of your components and what is your room situation like? Where are the woofers? I understand that it is easy to assume qualities of a component and then find out later that it was something else entirely that was the culprit.
What you propose will not work properly because the Vandersteen subwoofer design is unique. The Vandersteen x-over is simply a capacitor in line with the preamp which rolls off the lows to the amplifier (main speakers) at 6db per octave. The hinge frequency is determined by the value of the capacitor and the input impedance of the amplifier. The Vandersteen subwoofer adds a 6db bass boost to the signal it receives (via taps to the main amplifier) to compensate for the roll off. Other subs use the line from the preamp as input. They then employ a crossover with outputs to the main amp. This takes the bass load off the main speakers. The advantage of the Vandersteen approach is that the signal to the main speakers avoids switches, op amps, inductors, and what not in the signal path, not to mention phase problems inherent with steep slope x-over designs. FWIW, when one uses the Vandersteen subwoofer, it is a good investment to buy the very best capacitors available (MIT,Hovland, etc.), and install them at the input of the main power amp(s).
Maxgain: Regarding my complaint of midrange congestion ...
I own both Vandersteen 3s (being upgraded to 3asigs) and B&W Nautilus 805s. The 805s are very detailed. Instruments are well resolved and easy to recognize. The Vandersteens are too homogenized in the mids. I have to listen very carefully to identify instruments correctly. I am certain the Vandersteen upgrade will improve things, but I am not certain it will improve this problem. The Vandersteens are wonderfully coherent across their full bandwidth, however they are not as sensitive to the detail that my electronics are capable of delivering.

Samujohn: I had wondered how the Vandersteen woofers got the necessary signal after roll-off - now I understand. By the way, the REL operates from the AMPLIFIER outputs, NOT the preamplifier outputs like most sub-woofers. Cross-over frequency and volume are both adjustable at the sub.
Judit, are you talking about the sound of just the two speakers by themselves or when used with the REL's? Are they placed in the same spots when you make this judgment? I am sure I am not telling you anything you don't already know but the 805 and the 3 ARE VERY different speakers. You don't even need a high pass filter with the 805 as they are already rolled off 6db at 42HZ! Nice first order slope built right into the design! The Vandersteen goes much lower. Down only 3db at 26Hz. Still sounds like a set up issue to me!
I am talking about a direct swap comparison of the 805s with the Vandersteens without the REL inserted at all. I have run both speakers with chesky set up disc to optimize placement and room treatment. The muddiness in the Vandersteen mids is audible on human voice as well. However, remember I am running 10 year old Vandersteen 3s. Vandersteen claims significant changes with the 3Asigs, we shall see. I will let you know how this upgrade changes things, if you are interested.

The 805s have their own specific limitations as well - the roll off in the bass does not trouble me much at all. I am surprised by the quality of these small monitors. However, the tweeter is harsh and with some material - a problem that the Vandersteen's do not have.

Thanks for your interest - I learn quite a bit from discussions like these.
Judit, Have you gotten your Vandersteens back from the upgrade? I am wondering what you thing of the changes. I ssume that their will be some run break-in time that will be required. Be very careful in setting the rear tilt on Vandersteens, as this can account for what you describe as well as room interactions.

I am not familiar with the Chesky set up disc and wonder if you can tell me a bit about it? I know that some people rely on computer programs to position their speakers which in my opinion is flawed, compared to setting them up by ear.

You may find it interesting to read Richard Hardesty's issue #2 of the "Audio Perfectionist Journal", which can be downloaded from his web site for free.

My Vandersteens are not back yet from the 3asig upgrade. I am curious as well. Thanks for reminding me about tilt.

I find the set up disk useful for two things: (1) I am able to quickly assess maximum separation by moving my speakers apart until the central image begins to break up.
(2) They have a listening position called "beyond" the left and right speaker. The narrator's voice actually shows up to the left of the left speaker then to the right of the right speaker. I often find that these image positions are the ones that are most affected by room interaction. One side will image "beyond" while the other doesn't. I invariable find the source with the result that sound stage width is as good as it gets.