The truth on low efficiency speakers like B&W?

I've been learning a lot lately, but I'm still questioning the true amp requirements for speakers such as B&W 802s. I've seen mentions of using 45 watt tube amps all the way up to 600 watt mono blocks. Seriously, what is too little and is a 600 watt amp over kill? Can the speakers ever be pushed hard enough to overcome an amp like the Parasound 2250 with 250 watts/ch @ 8ohms and 400 @ 4 ohms? Is heavy bass music require the upper ends of power? Thanks. Owners please chime in with issues you have had with low power amps.
Yes heavy bass played loud taxes amplifier power.
The "45 watt tube amps" are being used to play normal music at normal volumes. period.
The 600 watt amps are for dudes with small dicks...
As compensation. (i guess)
The Parsound seems to be in the middle, or 'reasonable'
I can't really comment on 802s. I have 803d and 804s which I have driven with Parasound JC1s and Halo A21 amps.

Its not so much the total power, but the stability of the amp driving 3.5 ohm loads. Both the 803 and 804 (and I assume the 802) have a minimum impedance at low frequencies of 3.5 ohms (latest versions are 3 ohms). The A21 is rated for a minimum of 4 ohm loads, as is probably the 2250. At 3.5 you are stretching it. The JC1, on the other hand, is rated to drive 2 ohm loads.

To my ears, the A21 did not seem as open and relaxed as the JC1 driving the 803d. And I am not talking high SPLs where a more powerful amp would be needed. I am talking 85-90 dB where you are still under 50 Watts peak. While the efficiency of the B&W is not as good as a horn loaded driver, its not that bad either. I think mine are 88 dB at 1W, 1 meter. Newest versions are 90 dB, 1W at 1 meter.

You only need 600 W if you listen to really high SPLs. It has nothing to do with the size of your...
Got an idea what Elizabeth calls small but "normal" is up for grabs, so to speak.
Well you have to consider the crest factor of high quality recording.

Excellent orchestral recording have peak to average ratios that can exceed 20:1, which means that the average levels are 20 times lower power than the peaks. If you like to listen at an average of 85dB at your listening position, which might be 4 meters from the speakers, well the power requirements add up pretty quick.

Let's say you have speakers that run 85dB/W. You're 4m from them, so the level drops to 73dB (6dB for each time you double the distance, 1m - 2m - 4m). So, to reach your 85dB, you now need 4W instead of 1W. Now, the crest factor of 20:1 needs to be factored in, which puts the power required on peaks to be at least 80W. If you want your average level to be 88dB instead of 85, then you need 160W peaks. Any amp that puts out less than that will be clipping.

For clean reproduction from speakers that run low to mid 80s and normal listening distances, you should always pick an amp with higher power reserve. 250W would be my minimum choice with speakers like that.

Speakers that run near 90dB and above can get by with amps of quite a bit lesser rating.
No matter how powerful your amp is you'll never get bass from B&W 802. It's a unique speaker that doesn't benefit much from quality or quantity of amplification and it's a biggest looser in audio industry IMHO(pardon 801's bigger).
09-11-12: Elizabeth
Yes heavy bass played loud taxes amplifier power.
The "45 watt tube amps" are being used to play normal music at normal volumes. period.
The 600 watt amps are for dudes with small dicks...
As compensation. (i guess)
The Parsound seems to be in the middle, or 'reasonable'

Elizabeth, like I told you on our date at the beach, it was just "shrinkage"!

But seriously, I've measured 105db peaks at my listening seat on acoustic jazz played at realistic levels. I've also measured about a 5-6db loss between my listening seat and 1 meter from the speaker. Assuming a 4 ohm, 86db/2.83v/1-meter speaker is 2w at 86db, then 110db output at 1 meter would take 512w for a momentary peak. Some rooms aren't as lossy as mine and don't suffer much loudness difference at all between one meter and the listening seat, but some will suffer more. 105db peaks are, admittedly, very loud, and a lot of speakers can't comfortably get that loud anyway. Nonetheless, in a large room with a 4 ohm, 86db sensitivity speaker, 600w/ch is not entirely compensating for something. Of course, for Maggie owners this is an entirely academic conversation, because even a 20.1 doesn't sound very good at live listening levels. Or if your listening levels peak at, say, 97db at one meter, then a 45w amp will indeed work just fine. But a trumpet is louder on peaks than 97db at one meter, no less a rim shot on a snare drum.

Most of time I'm listening at 85db or less (mostly less) volume levels at my listening seat, and the highest peaks are in the 95-100db range, depending on the material. 45w/ch would be quite marginal, even under normal circumstances.
Whatever one's motivation, for better, complete bass at higher volumes, more power will only help and seldom ever hurt with speakers like the B&Ws.

If you only listen at low to moderate volume, you might get away with 40-60 good SS watts.

Tube amps + B&W speakers are not normally an optimal match from a technical perspective TTBOMK though some might still of course like the sound.
I've learned the hard way to look carefully at a speakers efficiency rating. A lot of people say buy the speakers and then the amps. This approach doesn't work for me. If you're like me and you've found wonderful tube amps that you just can't live without, then you basically want to find the speakers to match the amps ensuring that neither of their characteristics get in the way of each other. I know a lot of people won't agree....but that's my two cents.

Tube amps are not for every speaker. Tube amps will struggle with highly inefficient speakers.

My general rule of thumb is this: 90db and higher - you are generally fine with tubes or SS or Class-D. 89 db and lower, you better be looking solid state, Class-D or some hybrid (the new Rogue Audio Class-D/Tube hybrids have me intrigued.) That's just my general rule of thumb when shopping.

I'm 100% in the tube camp, so I look for speakers like Sonus Fabers which tend to have wonderful efficiency ratings giving me a plethora of amp options or in my case, letting me use my tube amps.

That's just my opinion. I've found the tube amps I love (McIntosh 2301's married with Gold Lion tubes) and now I have to find the speakers for them!

Thanks to Elizabeth I use a 10 watt tube amp. No need to compensate. :)
Also, haven't heard a B & W I liked overall.
I can lick my own eyebrows so I went to headphones lately. 600 watts isnt compensation, my 400w amps can shut down during loud complex music on VMPS speakers so 600 could be good if I could afford it.
N802's are not inefficient. They are specified at 90db 1W and 1 meter. They are however, very sensitive to the amplifier driving them and cabling. It is all too easy to end up with a lean, bright sound.

The W4S ST500 is an excellent partner along with a decent tube preamp. A VAC Phi 110 produced excellent bass also. The tube amps will produce a fuller sounding bass with less impact. The VAC pa100 sounds real entertaining with jazz.

The N802's are not the last word in bass definition or tunefulness but one can certainly achieve a good spectral balance and plenty of bass with the right setup.