Others have used 6V with no problems. The 6V batts will no doubt last longer and cost much less than using a 9V batt.
But there is a consideration here....
The 49-425 has a 5 volt output voltage regulator on it's PC board. As the batts drain down past 5 volts, the voltage regulator outut could become unstable and cause false pictures - this is common with the MS-20.
Now here's the catch.... a 6 volt battery pack (4x "AA"s, etc) will drain down to five volts well before the batteries are fully drained (fully drained 'dead' voltage for a typical alkaline batt is around 0.9 volts). So if you replace the batts when they are just below 5 volts, you're throwing away a large portion of their capacity! So you could use 5 or 6 x"AA"s (7.5 to 9 volts nominal), which would allow the batts to be almost fully drained before they need replacing. Doing this will lead to some extra power being 'wasted' (dissipated) across the 49-425's regulator, but the savings of being able to drain the batts farther should more than compensate for this waste.