Every time I hear any audio person tralking about making things 'look' right. I question that persons ability. Any genre. I have no problems with people checking things with audio tools. Stereo width, mono compatibility, maximum and minimum levels, spectral content - all great tools but there is a breed of person who believes that these things are templates. Rap must look like this, another look for old rock and roll and another for drum n bass, or other electronic dance music? I'd suggest that to even call themselves mastering engineers is a bit of a joke. They take people's carefully crafted recordings that already sounded good to them, and then destroy them. Let's be honest here. Did you give them the track because it needed mastering, or because you felt you didn;t have the skill to do it? In the old days mastering engineers cam with a barage of special subtle equipment that polished the already decent product, and had amazing speakers in a decent room. Now we have idiots with enough money to buy a plugin suite, who mangle music. Hopefully your person was the experienced, careful guy with decent gear and room? I assume the pain comes from turning it up to the kind of listening level your music needs - if this is the case, then they got it wrong. There will be too much of certain sources in the mix - typically constant sizzle from the percussion. Totally unreal so maybe a sound at 9.6KHz that has been boosted crazily, then burried in the other sounds. It won't show up in a squashed dense mix when you look at it visually. The 9.6 might be anywhere from say 7 to 14KHz, but is just a broad band splodge of sound - it won't be a spike that would show up - just one of those common brighter areas, but it's a killer to some people's hearing. If the mastering person is this bad, then his gear sounds like it might have a dip in the response around the problem frequency that they have not even noticed. You are the client, they work for you. Tell them there is something wrong and it hurts - get them to fix it. It sounds like you gave your project to a audiophool.
If a client wants something that is in my view, wrong, then I try to explain, but if they insist, then they are the client, paying the bill and they get what they want - UNLESS - I can afford to say sorry, we can't work together, so bye.
I have used a real mastering studio just twice in my career in audio, when the client insisted they always did this. I have to say that both times, they did things I had not thought of and the result was better. Both times, I pinched the thing they did and replicated it myself for other similar jobs. If you post it here, many people skilled in different genres will listen and give an opinion. I suspect they will all agree with you. Unless you have a hearing issue, then the person with one could be your mastering engineer?