The KM 184 is a state-of-the-art small diaphragm condenser microphone with a uniform, frequency-independent cardioid polar pattern. Its very natural, transparent sound with very low self-noise and high SPL capability have made the KM 184 a worldwide stage and studio standard for many...
en-de.neumann.com
Now, I don't normally put much store in mic speccs but Neumann I trust! (the mic also won a sound on sound award in 2000)
The graph shows the LF response to be only about 5dB down at 100Hz (and the guitar only goes to 80Hz) the 'presence peak' typical of almost all SDCs is a modest 5dB ish at ~8kHz and then a gentle roll off. I don't have a KM184 but that does NOT look like a "tinny,thin, shrill" microphone to me! So, what is wrong?
The first, VERY unlikely possibility is that you have a faulty sample. Easily checked by a voice comparison with any gash dynamic you have around, if perchance you don't have any other mic you can pickup a very serviceable dynamic for around $25 Behringer XM8500. Even one of the Neewer/BM800 LDCs will sound good enough to prove the KM out. (EVERYONE should have a spare mic, spare XLR cable and a digital test meter IMO!) N.B. I say a "vocal test" because very close, 25mm speech will all but eliminate the room from the equation. It will also eliminate any possible, but again very unlikely, problems with the pre amp.
That leaves the room, the guitar and then technique. My son records Bach on a classical guitar with a variety of mics. He is potless and dad almost (mainly due to supporting him) so he has only very budget mics. A Mackie LDC, a pair of Behringer C2 and a Citronics USB L*DC. He gets a very acceptable sound considering the extreme cheapness of his kit. One thing we did push the boat out for? A MOTU M4. Super bit o kit.
So, make sure the mic/pre is not the fault then listen to the experts here about recording techniques because there I leave you. Just a lowly valve amp tech.
*not really "L" actually about 20mm but it really does do a very decent job.
Forgot, son also has an SM57 and a FetHead , not sure if he has used that on guitar, will ask.
Dave.