First, on battery, several things here.
First of all, it seems that the closer terminals on the sr-71a appeared to be shorted together (From resistance measurement), so I think the battery should be discharged simultaneously and the source is 18v(edit: I now have reason to believe the two batteries are discharged differently.). However, as with all serial connected batteries, I think that once one of the battery becomes close to depleted, the voltage will drop below nominal voltage and would act as a load and effectively be driven to depletion by the other battery. It can be avoided by charging both battery in series(many factory battery packs are in series), or in parallel but by using reliable charger. It is for the same reason that new and old battery should not be used together, IMHO. I can be wrong.
Size. rechargeables are bundled together by 6 AAAA size battery sometimes. and maybe due to packaging, they are longer than non-rechargeable ones? duracell measure 47.8 mm. and tysonic 250mAh is 48.3 mm, radio shack is 48.5mm. Other sites have battery that are 48.5 mm. I agree that you have to squeeze the plate first (to the point the plate is flushed, and yes it take some force) before using the screws. It ease the screwing in and minimize the scruffing, although I think it is inevitable eventually. So my point is I do not have problem with the battery aferall(though I do take away the wrappers on rechargeables.)
Sound with 100 hours burn in(it sound good to begin with and i cannot tell the difference with and without burn in , cause i do not know what to look for). and again I do not know how to judge the sound w.r.t. other amps. so these are just my opinion on the amp (compare with without). But yes, it is fast, especially more noticable on bass, very fast. What appear to be a long thud from dap and laptop is now a snappy punch, very tight and locatable. I do not think there is emphasis on bass(i.e. eq should be flat), but the bass are now more defined and noticeable, and change my perception on certain music. piano also seems to be sharper, you can hear the "attack?" better. i use it on apple earbud and audio technica, cm7sv, e888, hd580 and some no name earbud. I will only comment on the fastness. apple and no name sound the same. with headamp, it is faster, but only by so much, i.e. bass still boom, e888 and cm7sv are way better (and that was where I discover the the fastness). 580 sounds great, just different from earbud, absolutely no complain there.
Ah, I also use er4s, I find that with ipod headphone out, it autually drive decently, but with headphone amp, there is a noticable 10-15% difference IMHO.
Overall, I like the amp a lot, because it allows me to hear what the source and headphone is doing and understand the music better. I do not have to worry "is it suppose to sound like this?..." (the part due to insuficient driving of the cans, i can now worry about the cans and source.)
I hope comparison with other amps will be posted soon, as I really do not know what precisely to listen to and what are the noticable difference, since I do not have another commercial portable amp.
