Why should the power consumption of a device mean that it would "whack out" more energy. It is all to do with the frequencies generated internally within a piece of kit and how well those frequencies were screened. A cassette player has no high frequency energy as a celluloid magnetic strip rubs over a magnetic head. There is bias on the tape, but that is still within the audio band, not radio frequency. A CD or MD on the other hand has demodulator circuits to "step down" the very high frequency encoded info from the disc into a range we can hear. That`s where the problem could lie. In comparison, a mobile phone deliberately transmits around 5 watts of UHF radio energy, hence the problem with mobiles on board.
Bob