A few months ago, I did a comparison of different feedback implementations for use with SE tetrode & pentode finals. ZERO feedback finals were also added to the test matrix.
The measurements were taken with ARTA STEPS at 1W power output level using a SE DC 6V6/6AQ5 amplifier as the device under test. A 1KHz test sine wave was used as the input test signal.
ZERO feedback tetrode mode : THD = 3.09%
Global feedback from OPT to driver cathode: THD = 1.60%
Local feedback at finals : THD = 1.60%
Local + Global feedback : THD = 1.12%
There are a lot of fears about the use of feedback within the SE amplifier building community.
For SE DHT finals, the purist would avoid using any sort of feedback and I would tend to follow that approach myself.
But with SE tetrode & pentode finals, the THD vs. output power curves are very poor as are the bandwidth measurements without using some feedback mechanism.
I did some listening comparison to the SE DC 6V6/6AQ5 amp with different feedback modes listed above and to ZERO feedback finals with my Altec horn system.
My personal preference subjectively is to use some feedback and carefully adjust by ear while documenting the effects on THD, bandwidth and gain. It is very much a system dependent process.