In this case there is no boost at all, only attenuation.
The efficiency drops by about 5dB.
The highs are attenuated by the felt and the rest by the filter.
The filter only works when driven from a low impedance amplifier but most pirates fear one >10 Ohm (supposedly damping factor fear).
Yes the amplifier must work a little harder to reach the same levels and yes that may increase distortion of the amp somewhat.
I can still play it sufficiently loud from Sansa Fuze so high power isn't needed at all.

Will be working on another mic pre-amp in due time.
The one I have no has the needed compensations and this increases the noise floor in the lows (did not use the optimal op-amps for pre-amp) and this higher noise floor (-40dB in the lows) obscures the THD measurements.

A K550 (and TH-02) are on their way to me...
Can compare/filter the K550 as well, I suppose they are more similar than different.

Dogears wanted to see measurements of the K550 with SRH-940 pads fitted.

see below:



blue line = K551 modified (left channel only)
purple line = K551 with filter + 3mm felt + SRH940 pads.
It should be noted that the pads are too small but with a little bit of stretching they might well stay on there.
For the test I taped them down.

At a first glance the bass levels would be too low but things aren't as they seem because the K551 here has a filter that lowers the amount of bass.
Fortunately the filter action is the same as the 940 pads do so...
Below the 'stock' K551 where you can see the 'warm' slant in the lows.
When you 'subtract' the purple line you will end up with a fairly flat FR.



When you use a stock K550, use 3mm wool felt in front of the driver and fit SRH940 pads (very tight fit ?) you may well end up with a very linear FR without having to use the electronic filter.

good find dogears !