I think that a "dynamic" speaker should refer to one which (1) tracks well the moment-to-moment changes in volume of an individual instrument/voice, as well as groups of instruments and voices, (2) does not compress the momentary peak volume changes in music, and (3) subjectively sounds this way both at very quiet playback levels and at louder levels.
It is not for nothing that the Quad ESL tops The Absolute Sound's 2010
list of the most significant speakers of all time. The Quad 57 also tops a similar list published by TAS recently, not yet available online as far as I could find. Full-range electrostats are hard to beat in the midrange and highs in terms of apparent clarity, detail, and dynamic responsiveness. In the mids and highs, they are "quick" to respond to micro and macro changes in the dynamics of individual instruments and entire ensembles, within reasonable SPL ranges they don't compress the dynamic peaks, and due to inherently low levels of distortion they also sound nearly as dynamic at quite low listening levels as they do at louder levels, a feat which few other speakers can manage.
The problem area for "small" full-range electrostatic speakers like the Quads is the bass region. It is in the bass where such speakers run out of excursion capability at relatively low peak volumes, causing problems in the reproduction of bass-heavy music of any type when the average SPL gets loud. If you want to listen to heavy rock or orchestral power music at peak levels over 90 dB (and even that level is quite optimistic), Quads will either be damaged (the ESl 57) or kick into electronic limiting mode for self-protection in the later models. This dynamic range limitation may cause some listeners to categorize the Quads as un-dynamic even though they handle midrange and high frequency dynamics very well indeed. The limitation on bass dynamic range is really the only reason why I (who have owned MANY loudspeakers in my audiophile life) have never owned Quad speakers in any version, even though I have often been mightily impressed with their performance on appropriate-for-them material.
If you are the type of listener who never wants/needs to listen at peak levels higher than the mid-80s dBs (and there are many music lovers who feel this is plenty loud enough for home listening) then even in the bass range any version of the Quads are quite dynamic sounding since they are just as quick to respond to dynamic changes in the bass range as they are further up in frequency, and since their bass distortion--when not pushed beyond their limits--is as low as most any dynamic woofer and lower than most. Just recognize than most Quad speakers in most rooms really won't put out much bass energy below 40 Hz or so. But then most music doesn't really need the bottom 20 - 40 Hz octave to sound natural; even bass guitars and drums only go down into the 40s.
Quads, even the ESL57s, can often surprise you with the material which they WILL play well. I remember hearing a dealer demo of the LP of the M&K RealTime recording of Earl "Fatha" Hines, the one with duo piano and tuba. It sounded extraordinarily realistic with all the piano left hand bass and tuba bass you could ask for at realistic concert levels.
