I've had 2 DL-304's over the years and used them with SUT's, Head Amp's and normal MC inputs. It generally works through anything as long as the phono stage has enough gain/sensitivity.
Sugden integrated has 100r input impedance and 0.15mv sensitivity so all ok.
QED battery head amp has slots for loading resistors (470R open) and capacitors.
This then plugs-in to MM phono at 47k. Think I used 100R/47R resistors fitted which gave actual load of 80ish and about 43. Haven't used it for years but did work well with very high sens. Switch on front is X24 or X50 so would give 5-10 mv into MM stage with 0,2mv in.
QED gave calculation as: 100x470
470-100 = 370 = 127R for 100ohm load, so 120 is nearest.40x470
470-40 = 430 = 43.7R for 40 ohm load, so 47 is nearest.
Audio Innovations 800 SUT is the best though. Not exactly sure what step-up ratio is but it was designed for the Audio Note IO which has much lower output then the Denon. I run it with load of 270-330k resistors (switchable on pre-amp) and has 2 secondary windings. I think it gives approx. 38 ohms(lower winding) and 50 ohms (higher winding), when using the higher (330k) load at pre-amp.
The lower setting 150k, (or 270k across 330K), is ideal for ATOC5 and gives about 10/22 ohms which is spot-on. The AT has twice the output of the DL304 so plays a lot louder, but Denon still has plenty of output if you load it fairly high (with resistor after SUT).
Another thing I found was that some cables can roll-off the treble due to high capacitance! Not many do luckily but it's worth knowing.
Interestingly it was quite well received in this month's HiFi World MC cartridge test, despite being 20 years old!
Note to ABRIL: have you seen this month's Olde Worlde feature?