OK... answered my own question!
There's a free utility called MacMP3Gain which can adjust MP3 volumes (but no EQ) without losing the lyrics tags.
Unfortunately the newest version needs MacOS 10.7, and I'm running 10.6.8. :( (and no idea if it runs on newer OS's)
So the old version has a very convoluted way of adjusting just ONE audio file at a time, there is no way to set say +2db, -2db etc. It sort of analyses the max volume of a track and then you have to give it a 'target' db, but the 10.6.8 version doesn't tell you the analyzed volume! So it's a bit hunt and peck until you get the amount of adjustment you want. Plus, it's all in integer adjustments in the 10.6.8 version. You can't ask for 89.5db, it's 89db or 90db. The 10.7 version seems to offer a lot more control.
There is a PC version which several people have told me works pretty well. To be honest, it is designed to take a whole folder of MP3's and make them all perceptually the same volume, but for me, I need to make sure loud songs are loud and quiet ones are quiet (the idea is to not have to adjust the volume knob as I do different songs), so I'm not really using it as designed. But it IS at least working without stripping the lyrics, so, 'mission accomplished'!