If I had a variac I would use it, better safe than sorry and it can't hurt anything.
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A cheap way to do it is to power it on for only a moment. Say less than a second. That will allow some A/C juice to get in and start forming the caps, but not enough to make them 'pop' if in bad form.
Them, after maybe ten minutes, repeat and leave on for say two seconds. Same thing wait for about ten minutes, then turn it on. It should be fine with this preconditioning.
If this is an original MX 110 and the filter capacitors in the power supply have not been replaced, I would be careful running the preamp- startup on a variac and take and hour to bring it to full voltage- but I would regard its use as very temporary until the caps are replaced.
The power transformer is what is at risk, but certainly in any event the preamp will not sound its best, even if it seems to sound OK.