^The sub distance is affected by the plate amp and built-in electronics which adds to the group delay. So using physical distance between the sub and Main Listening Position (MLP) wont be the right method. If you use the Audyssey mic in the auto-calibration step, it will take the entire group delay into account and give the right distance.
When you adjusted the level of the sub plate amp gain, you want to be in the negative territory such that after after any manual change such as bumping up the sub level on the AVR, the end result is close to 0.0dB. In your case the post Audyssey level of the sub should be at least -3dB so that after manual adjustment of +3dB, the end result is close to 0.0dB.
Did you do the sub crawl test to check the best positions to place the sub? If not, it's a worthwhile endeavor since you've only got a single sub. Note that multiple subs will help smooth out the bass, provide a larger sweet spot and lessen the impact of localization due to a high crossover frequency.
Also, if you've moved the speakers/sub and/or the MLP, it's best to re-run the Audyssey auto-calibration to get the new levels and distances. Did you use a boom stand mic or tripod to carry the auto-calibration? The boom stand mic will give a more consistent and repeatable result.
Think I found the sensitivity of the ADS L400
bookshelf speakers: 90dB/W/m with maximum power of 75W. The frequency range is 60Hz to 20kHz (+/-3dB), so the crossover of 100Hz is about right.
I did a quick check using the following online app
to determine the SPL/power requirements based on your reported numbers for the pair of ADS L400 (1983-1986). Using 6' distance, 90dB/W/m sensitivity and the speakers away from the wall, 75W per channel gets about 106dB at the MLP. The target is to reach 105dB. One only needs about 53W per channel to get the peak of 105dB.
The Marantz SR-5008 can easily deliver this power as it's rated to 100W/channel into "solid" stereo mode. In particular that you've enabled bass management by setting the fronts to SMALL and implement a 100Hz crossover frequency.