SEP as a term is new to me, but as long as we agree it means Single Ended Pentode that's fine with me. The reason that most single ended amps are triode is to get the low plate impedance which gives some damping. Typically one can achieve a damping factor around 2-5 in a good design without feedback. Six dB of feedback will double the damping and reduce the distortion to half and I think there is some merit in doing this for some applications. However many like the sound of an open loop amplifier and I do make the EM-7 that way.
There are some single ended amps that use pentodes connected as triodes and they behave like triodes on paper but a pentode is not designed like a low impedance triode and when triode connected may not sound like one. I have experimented with the popular EL-84/6BQ5 as both a pentode and triode and made a nice 4 watt/channel integrated with it which I would produce for around $1200 if there was interest. In that amp I use the tube as a pentode and enough feedback to get the damping up to some reasonable value and likewise the distortion low enough to sound good.
If one makes a pentode amp with a 6CL6 (and I know that tube well) one can do it in one stage (one tube per channel) as we see in the picture above from TrungT. A high transconductance pentode like a 6CL6 has enough gain on its own to be driven by a preamp or CD player. Connected as a triode the gain is much lower and a second stage is required. Nice looking amp but I would guess the damping is less than one. A pentode has very high output impedance and without feedback one is stuck with that along with a large amount of distortion.
I am currently working on a 10 watt push-pull amp using the EM7 tube and I like it very much. Without feedback it has good specs with a damping factor of 5 and half power distortion of about 2%. I am considering putting a transformer output tap selector switch with easy access to the user. This switch allows some very interesting options to be readily available to the user. I have often suggest that owners of tube amps with multiple output taps try all of them and choose the one that sounds best to them. There are large differences with most speakers and "matching" the load is only necessary when the full power of the amp is needed. A matched load is not always the best load as those of you have read my paper on light loading now know. This tap selector currently has 3 positions which vary the transformer ratio over a large range and thus vary the load that the output tubes see which affects them greatly. Most of what people think about optimum loads for particular tubes is incorrect. There is no simple answer to that question. The optimum load is a function of many things, most of which have little to do with the particular tube and more to do with the application.
I think the use of low impedance triodes used in push-pull has been overlooked and most of the emphasis has been on single ended. The problems with single ended are many, the worst being the output transformer which has to be gapped to support the unbalanced primary current. Most makers of these amps don't dare mention how bad the distortion both harmonic and intermodulation is at low frequencies. One need only look at John Atkinson's measurements of SET amps to see the rising distortion in the region from 100 Hz down to see the beginnings of transformer saturation. A test I would like to see made is the traditional 60/4000 Hz intermodulation test as only this test shows how the bass is going to modulate the mids and highs in a very objectionable manner. It appears to me that most SET users listen to music with little bass and perhaps they are the best owners of those amps as the intermodulation problem will not occur in their listening.
I would be interested in seeing some links to push-pull triode amps using real triodes like the 6EM7/10EM7/13EM7 tube, all of which are the same but for filament voltage. This tube is my choice as it has the lowest plate resistance of the lot, good dissipation and is readily available. In comparing it to the popular 2A3 I found it similar in all the important parameters with the further advantage of a indirectly heated cathode which spares us the problem of hum due to heater supply.
