I love the inability to read...So with Stax at home in the big rig, there is no need for large phones - which the AT A700s certainly are. For good quality small closed phones I'd say the candidates are:
Aiaiai TMA-1 (DJ phone, fairly dark, recessed highs, mid emphasis)
AKG 171/172 (Home and DJ versions, a little bulky, bass light, massive soundstage)
Audio Technica ESW9a (wooden cups, very mellow, excellent timbre)
Audio Technica ES10 (very good bass for a portable, slightly overwhelms the mids and treble)
Beyerdynamic T50p (typical beyer signature -bass light and treble tilted up, very good instrumental seperation, early models suited larger head sizes)
Beyerdynamic DT1350 (atypical beyer signature, good bass and neutral treble, very much FOTM on headphones sites, but with good reason)
Phiaton MS300 (neutral in both the positive and negative senses, inoffensive but also unengaging)
Phiaton MS400 (warm slightly overblown bass, good prat)
Sennheiser HD25-1 (the old trooper, indestructible, somewhat restricted at both ends of the frequency, prat in spades, suits the broadest range of music)
and the B&W P5 (fairly dark, restricted top end, but very good commuting phones0
I've listened to all of them and own more than a few (AKG 171, the two Beyers, the Senns, the Phiaton MS300). Generally the sound quality of them all will be several steps higher than the Px100 and several steps lower than the Stax, the sound signature differs quite a bit and would probably need to be considered in light of your unstated tastes in music.
As a left field suggestion, Stax used to have a portable electrostatic the SR001, which came complete with it's own battery powered amp (about the size of a box of cigs). Whilst it was recently discontinued, it may still be available from some dealers. It is the only portable phone I have which can replicate the timbre and decay of an acoustic bass. Like a lot of these models though ultimate extension is somewhat compromised.
regards,
Giles