Vandersteen high-pass filter with other Subs

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!