701's can and are in my case, very well even with batteries only.....not all small portable amps can do this well at all.
The O2 and 701/2's is not an issue.
I have three transformers of all sizes and have switched back and forth and compared to just batteries and there is no audible difference to
me at all.
I ususally listen to just batteries until they need charging and charge overnight and start the process all over again.