using high end speakers wit integrated recievers

Is it possible/advisable to power (and appreciate) speakers such as the B&W 800 series line with an integrated amp, such as the Pioneer Elite VSX-21TXH?

What are the drawbacks?
Mainly the entire sonic range will be lacking and you won't appreciate what the speaker is capable of. Years ago many audiophiles used 800 matrix series with Adcom and although very good but still far from the potential they can deliver. If you are planning on upgrading and building a system then you won't be sorry.
It depends on the amp. The large Musical Fidelity ones, NuVista, TriVista, 500 and 550 would do a very good job. The Krell 300, the new one that retails for $2500, should also do a good job, it is surprisingly powerful. Gamut also makes some good ones. What you will find that the good ones are not especially cheap or small. The MF ones weight 110 lb., thankfully divided into two units. They are in another class entirely from Adcom and the like. When I was a B&W dealer I drove the 801s I demoed with a Crown SL-2 (?) , which could pulse 1600 watts. High power and especially high current capacity is necessary, you can find these in an integrated amp but you have to look carefully. There was a MF 500 for sale on here recently for $2900, I think this would be a good choice both for value and performance.
I agree with Stanwal on his suggestions, however, the OP speaks of using a Pioneer Elite home audio reciever, which IMO is bad idea. It's kind of like buying a Porsche 911 with an engine from a Ford Taurus.

Been there, done that.... there are not many shortcuts to great sound in this hobby. You get what you pay for.
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Normally I would totally agree with the previous posters on this one but recently I have had first hand experience similar to what you are asking. I have been using a Pioneer Elite sc-27 receiver which is the big brother of the receiver you mention, both using class D ice modules albeit with different power ratings. I've used class D ice modules before in a Rowland Concerto integrated and was not terribly happy with the sound of the technology at least in that product, dry sounding and lacking air, but I wasn't terribly concerned about it since I bought the receiver for surround sound. I have been feeding it straight from my Apple TV (Apple Lossless files) using toslink all connected to Definitive Technology STS's although briefly with JM Lab Mini Utopias (The Minis are still some of the best speakers I have ever owned).

Now used in the PCM Direct stereo mode with the MCACC auto room correction turned OFF I have been truly surprised with the quality of music over the system (MCACC goes back ON when watching movies). I can not offer an opinion using an analog signal into the receiver though, but used as a D/A converter and amp I was totally surprised. Very revealing and very detailed and not fatiguing. With the JM Labs and the Definitive Technology if the volume is way up the treble can get a tad hot with this combo but that may be more a characteristic of the Def. Tech's than the Pioneer Elite.

Is it better than my old Mini Utopias, tubed integrated and separate D/A converter? Not quite, but I was very surprised with the sonics. That system seemed to be more delicate, tad more three dimensional than the Pioneer Elite but we are talking 2 grand of receiver vs 7 grand of tubes and D/A converter. You'll just need to figure out where diminishing return kicks in for you.

What would be the drawback in your system? Most likely power or lack there of. I'm not sure if you can bridge channels with vsx21 (I'm thinking around 100 or 110 watts per channel) like you can with the sc-27 (140 watts but bridgable, although I don't bridge them) but probably ultimate control over the speakers. I've had a little experience with B&W 802's and just from a sonics standpoint I found them to be quite smooth in the high end and full bodied but they did need power (at the time they were hooked up to a Mark Levinson stereo amp). If you were to get into trouble with the combo again I think it would be power and controlling the woofers but ice modules tend to control woofers quite well up to their power limits. If fed a digital signal and kept in its power band I think you might be surprised with the clarity. If you can swing it get one and try it out. Probably get it for under 800 bucks. Is it advisable? It's not like the receiver is going to give your speakers a venereal disease, if you don't like it, sell it.

Please don't misconstrue this as saying the Pioneer Elite is the best sound you can mate with your speakers. In fact I would think there is some sort of "circumstance" that is forcing you to consider going in that direction. Like the others have said I would not have ever considered 800 series B&W's with ANY receiver. It's just in my experience that Pioneer Elite line is very good for the money. But then again I test drove a Taurus SHO a few weeks ago and was impressed. Go figure.
Arch2 - the new Taurus appears to be quite and upgrade over the previous generation... especially once you move up to the SHO line, it's basically a muscle car for folks in their 40's.
The SHO is a muscle car for someone in their 60's.

Straight line = good
Not so straight line = not so good
panic stop = panic stop (frighteningly bad brakes)

Big on the outside and little on the inside (how did they do that?)

My son and I went to look at one and after a (short) test drive left laughing at how arguably America's best new car builder could have missed the mark so bad.
Thanks for the SHO car evaluation "Snofun3". I think there is a Car and Driver forum somewhere to post for those interested. I have read lately that some Toyota's speed up or don't stop at all when you don't want them to, and even brought in NASA to help. Now let's get back to integrated recievers and audio.