Zaphod Beeblebrox

If  single 57's present a probematic impedance load for amplifiers to drive, then is it correct that driving a two up stack is going to have the amp seeing a load that is halved, putting the amp under even more strain?

Correct.

How is she taking the output from the phone at the moment, dock or stereo jack? As non-ideal as using an iPhone as a player is, she should still be able to get acceptable results even from MP3s. I can if I use a dock to RCA cable. 

That was my original suggestion (to dump the iPhone and use the lappy, via the headphone socket). She has yet to report results.

Rip her CDs to Apple Lossless or AIFF (I prefer AIFF) or WAV Use XLD to transcode yr FLAC to AIFF/ALAC/WAV You can sync the files with the iPhone. Use a digital iPod dock that bypasses the iPhone DAC and sends out a SPDIF. If you are using a Mac to play music, download Audirvana Plus or BitPerfect. Sent from my iPad using Tapatalk HD

Thanks for the suggestions.

If she is buying music on iTunes then I suggest MOG. 320k streaming via wifi, all the music you want and for the price of 1 CD per month. I am still amazed at this service.

VERY impressive. A 14 day free trial seems like a good plan.

Does she have the setting enabled in iTunes which reduces the quality of media synced to the iPhone  (to save space) ? .... I think the default is quite compressed (128kbps?).

Even at 180kbps, sound quality is noticeably lacking (her system is quite high resolution).