I believe Mercury were generally single mike position with one continuous take, and once the tape started rolling, they didn't touch the mixing board. That makes for a very natural soundstage and dynamic, but it also meant that they couldn't control overdriving in peaks. You'll find that Mercury sometimes has points with much higher distortion than Living Stereo. That may be what you are hearing in the Bartok.
Tympani are generally the toughest to reproduce because low frequencies require a lot of power to produce at high volumes. If a system just can't reproduce the low end, like your KLH perhaps, it isn't a problem. But better systems are going to immediately point out imbalances in subwoofer levels or lack of sufficient wattage to push the loud low frequencies. If you just have a two channel system without any subwoofer, it's doubtful whether you will be able to reproduce that accurately at all. The super low frequencies are going to run roughshod over everything else and swamp your speaker. But with a subwoofer, you can hear everything in proper balance, because the hard to reproduce stuff is ported away from the mains to the sub. I use Mercury's 1812 Overture to balance the subwoofer in my system for this reason. The huge low frequency cannon shots are very unforgiving to systems with the sub turned up a little too high, which most people do. Not all recordings contain sub bass, so people think they need it louder than they actually do. However, if you adjust your sub to the point where it is just barely able to reproduce those huge low blasts in Dorati's 1812 without rattling, the sub bass will fall into proper balance on all well balanced recordings. It's an easy way to calibrate to something that pegs 10 on a scale of 1 to 10.
Living Stereo often used three mikes recorded as separate channels. They would mix down the three channels to a final stereo master, and I think they were able to perform edits between multiple takes on the fly, which you can sometimes hear if you listen closely. This added a generation to the master (not a big deal) but gave them a little more control over peaks and gave them more takes to choose from. You don't find the huge dynamic swings and distorted peaks on Living Stereo that you find on Living Presence.
These old recordings are not noise free and Mercurys often have distortion in the peaks. But the thing you do find on these old early stereo records is very simple direct miking and a stone flat response and a wide range of frequencies. This was early on in hifi, so the engineers were very aware of their response curves. They used mikes with very flat and very broad range response and pretty much put them on tape without any equalization. Compared to a lot of modern recordings, these are the most direct and accurate you can find.