ok, so a few things to consider here.
- These specs are almost always at 1kHz. As you change frequency the power requirements will change and can end up being quite a bit for lower bass frequencies (luckily the impedence of a planar is flat so its just a sensitivity curve). This part ultimately isnt all that important to overall outcome of a can/amp pairing IMO (I have some pairing I love that are theoretically vastly underpowered), but it is something to keep in mind.
- Power of an amp, generally speaking, is peak power @ a certain distortion level and not a continuous draw. To build on this, as a power supply starts reaching towards its maximum current output (current supply of an amp is largely determined by power supply) all sorts of weird things can happen. You can get very quickly rising distortion, molasses slow slew rates, bad over shoot, and really just run into a huge number of issues.
- Some amps just really don’t like lower impedance cans for one reason or another (and I’m not just talking OTLs here). LTA make good products so you shouldn’t run into any stability issues in this specific circumstance but it can happen depending on topology. More importantly though, its not too uncommon for amps to be unoptimized at impendences this low and just have some measurably wonky performance due to a number of reasons (bad damping factor and unexpectedly low gain are two big ones).
- Honestly, while going “I need to meet the specs at 120dB for proper over head” isnt a bad rule of thumb, it just doesn’t always apply. A great example of this is “tube power” where speaker guys will often claim that tubes have a higher “equivalent power” to solid state. While I sometimes feel that even the 13R doesnt give me satisfying kick on susvara, the HPA-01M (at 350mW @ 30 ohm) absolutely never leaves me feeling that way. You also run into cases where the step between amplifiers is so large that the “under powered” one ends up subjectively a better experience (BX2+ vs Mass kobo 428 on diana V2).
Over all, I think eyeballing power is fine and useful but its not the whole story. Amps can be a bad pairing for a number of reasons even if we ignore presentation synergy issues, and these issues tend to be exacerbated as you go lower and lower in impedance. By all means give LCD-5 a shot on the MZ3. We wont know for certain if it works well or not until a few people do. Its just that if you asked me to make a bet on if it would work or not, I would put my money on ‘no’ due to the mz3 not doing great with other more demanding planars