What kind of mixer? Are you padding the inputs on the mixer? Basically, you are running a +4dBm line level output from the Nak (nice cassette deck) into a line level input designed for either +4 or -10dBm. BTW, I may be wrong about that XLR out on the Nak, check what it says on the unit. I used to use one of those many years ago, I believe that it's +4, in your case it would be best to be -10, -20, or better yet, -50 dBm. In any case, please fiill us in.

If you have line inputs that are balanced (TRS) just use XLR>>TRS cables into balanced line inputs and use those. You didn't list what console your using but most have both connectors XLR/Line inputs on any given channel.
Using the Mic inputs without pads could overdrive the channel preamp and it's difficult to control the level. You'll end up with the trim/gain control all the way down. If your board has the typical mic/line switch or separate TRS line in jacks they already have the proper pads built in and will handle the cassette signal better and with more control when setting levels.

You'll want to avoid mic pre's entirely, if possible, for this sort of thing, so use TRS adapters and use the line inputs whenever you can for the Nak. Chances are, you could be sending too hot a signal into the mic pre, and possibly overload it on transients & spikes, even if your levels look right. Some of the better consoles have an entirely separate gain stage (and IC amp) in the mic pre that sits ahead of the line input. Other's just do it with resistors/pads, etc. Mostly it's just about proper level matching, but it's a good rule of thumb to stay out of your mic pre's for line level routing.