I own MB-185's, powering Thiel CS 2.2's (not the demanding load that some associate with the brand - 8" woofer and fairly flat 4 ohm impedance with well-behaved phase angle, average sensitivity of about 87dB).
My general observations about triode vs. tetrode - the inevitable variations in recordings that cause disk-specific preferences aside - go like this:
Triode: Superior in only three areas, but three of the most important areas - tonal balance, harmonic structure, and audible distortion (within its dynamic limits) on transients.
Tonal balance: Triode has a more neutrally-balanced tonality throughout the heart of the audio range. Tetrode is is brighter and less evenly-balanced in comparison, with a relative lack of lower-midrange/upper-bass fullness combined with an overabundance in the 'presence' range of the upper-midrange through the low-treble.
Harmonic structure: Triode operation reduces odd-order harmonic production, while the push-pull operation takes care of reducing even-order byproducts. Tetrode produces more odd-order byproducts, which push-pull cannot cancel, so they will heard predominantly over the more pleasing even-order series. This skews the amp's harmonic signature to yield a 'harder'-sounding emphasis with tetrode, while triode gives a more natural descending harmonic distortion with asceding harmonic order, and lower total distortion overall. This is entirely independent of the tonal balance issue noted above, which can be addressed to some degree through choice of input tubes, cables, etc. Triode will always have the advantage over tetrode when it comes to natural sweetness of harmonic presentation.
Transient distortion: Although triode won't give the dynamic impact of tetrode, and its limits are lower due to the reduced max power, transients are cleaner distortion-wise, even though they are slightly compressed by comparison to tetrode. This means that triode will deal better with sibilants and sound smoother-textured overall, while tetrode will give some false emphasis to fast-rising impulses that produces a slighly 'edgier' sound, with more emphasis on 'detail' than is strictly natural. BTW, this is just my listening impression, not something I know from a technical standpoint, and I surmise it could be the result of either intermodulation or temporal distortions.
After those three areas, I believe tetrode holds the rest of the cards, including those relating primarily to all qualities physical:
Greater extension: Tetrode has more extended response at the top and bottom of the frequecy range.
Bass definition: Tetrode maintains greater control over woofers for more bass-range detail and tonal differentiation with less overhang, although the bass often won't sound as full-bodied in tetrode as in triode.
Dynamics: As mentioned, triode will compress earlier and more than tetrode on dynamic peaks. But tetrode is also better at eludicating subtle dynamic shadings of expression on the micro end.
Soundstage: Tetrode presents a clearer, and more defined and separated soundspace than triode, which by comparison sounds somewhat homogenized and foggy. Tetrode also has greater scale.
Imaging: Tetrode presents better-focused images, triode more amorphous ones.
Resolution: Tetrode is better at illuminating decays, and combined with its clearer atmosphere and better micro-dynamic tracking, the result is more transparency to fine detail and more precise articulation.
In sum, I think the amps pass more information in tetrode, but don't necessarily present it as nicely as in triode. (Of course, all my observations are drawn by me in my system, and are therefore not to be taken as universal truths.) I do end up using the switch as a tone control sometimes, dependent on the recording. If a recording is overly-aggresive, I'll likely go to triode regardless any impact or definition I give up. Likewise, if a recording is muddy or hooded-sounding, I'll use tetrode to help pry it open, harmonics be damned.
With well-recorded material, its a little more disconcerting to me to have an amp with two distinct sounds, neither of which I know is absolutely correct. I'd love to be able to combine the purity and naturalness of triode with the control, power, extension, and precision of tetrode. But since no amp is totally correct, I guess it can't hurt to own that at least gives you an option. So on decently-recorded disks that won't penalize you too much either way, my preference is mostly determined by the type of musical material, where its inherent priorities lead me.
BTW, I often do my comparisons on the fly, sometimes with music playing. Over the past year, switching with impunity has possibly cost me two output tubes and one output fuse, but I can't be entirely sure of the causes. One thing I know is that if you are going to do this, it is vital to flip the switch as quickly and positively as you can - no half-assed indecisive or scared-acting switch-flipping. Just do it. Or don't do it - this has got to be a personal call. I've flipped hundreds of times with no problem, but who knows what'll happen next time? Oh, and to make valid comparisons, you MUST compensate for the 2-3dB volume differential between the two modes (something that's easy with my Levinson preamp's programmable muting function).
As a final thought, it's worth saying the change in modes is not a night-and-day affair - it can be pretty subtle, or pretty insignificant with certain sources. After all, most of what makes the amp sound like the amp that it is, remains the same in either position. Right now, I'm considering a coupling cap upgrade and some experiments with input tubes in order to try and bring tetrode closer to my ideal tonal balance, which for me is its biggest flaw at present.
So far, I'd say that I run the amps 80% in tetrode over triode, but when I first got the amps, the switches had been removed by a previous owner and left set on tetrode, so for almost the first of a year I had them I never heard triode. As I've gotten used to the restored switch and its effects, I find myself increasingly willing to forego some of the qualities I had become accustomed to before the repair and listen in triode.
