You guys got me thinking... Perhaps it would make sense to have a simple circuit that would make it possible to use regular DMM to measure linearity of the cartridge/preamp output, using test records. Nothing unheard of, precision rectifier followed by a low pass filter would probably do the trick - get accurate DC output proportional with the amplitude of the test signal. Of course, devil is in the details - there is a good reason why wide bandwidth accurate voltmeters cost enough not to be part of a typical audiophile's toolbox of tricks. But I'll think some more tomorrow, it's been a long day for me....