If you feel like playing with an old '81 but then remember how unstable the 16K RAM pack was, it's fairly easy to replace the 1K internal RAM with a larger static RAM. Here's one description of how to go about it.
http://blog.tynemouthsoftware.co.uk/...l-16k-ram.html
That will give you the classic, clean external looks of an unexpanded '81 combined with the rock-stable 16K of internal RAM it really needs to run any of the classic software.
I experimented with using an MP3 player (rather than a cassette player) to load files into the '81 and it didn't work initially because the maximum audio output level available from one stereo output channel of the player (a small one which runs on 1.5V) was not enough.
To get it to work I had to make a special stereo version of the file with one channel inverted, thereby using the left+right channel outputs in an antiphased 'bridge' configuration to produce double the output voltage swing (measured between the two channel outputs).
I didn't connect the player's common output ground to anything. Instead, I connected the player's left channel out to '81 tape signal-in and the player's right channel out to '81 tape ground.
This arrangement works without any problems as long as the player's own 0V/Ground is completely isolated from ZX81 0V/Ground, which in the case of my battery powered player, it is.