from Harvey Gerst's famous Marshall mic review:
Marshall MXL-600 $270 Veiled top end and exaggerated low-mid, compared to the Oktava MC-012. About 1 dB lower output than the Oktava. It just sounded very dull and lifeless. Very easy to bottom out as well.
Marshall MXL-603 $99 This was a flat-out winner, folks. Almost identical to the MC012 in sound, with a wide cardioid pattern, almost approaching omni. We used them as drum overhead mics, and they did a great job. The diaphragms are easy to bottom out on voice, but with a pop filter (and positioned above the singer's mouth), they wouldn't be bad as a vocal mic on some singers, and they'd probably do fine on acoustic guitar, and many other instruments. They were also a perfect match to the Oktava MC012 - they sounded nearly identical.