maybe i started skimming over things, i didn't see any heated debate, just different ideas
i've got a relatively small 29 gallon tank, an HOB filter, (only foam blocks, replaced everything else) just for surface area, i could possibly want to use rock wool which would have even higher surface area. other then that, the foam block on the front (prevents fish from getting trapped) is the only real mechanical filter (only place that accumulates crud). the only gain substrate has on better/greater bacteria for processing the nitrogen cycle is surface area, ... as has been mentioned, if your using gravel, depending on your filter system it may have a higher surface area (and in turn more bacterial culture) if your using sand, then the substrate will always have a higher surface area), if it's not cleaned, even higher surface area in your substrate, ... but the good bacteria isn't only in the substrate, it's on every surface in the aquarium.
i've heard mention that some of this beneficial bacteria for the nitrogen cycle does it's job in the dark, so that's something else to look into, ... inside most filter systems it's dark, so this isn't going to impact most filter systems, it's also only something i've heard once, so it's only a curiosity at the moment and not something i have to rethink my tank around
heavily planted, the substrate is bound with roots i'd rather not disturb, detritus & mulm have saturated the substrate, i have NEVER cleaned it. i don't prune the plants till they are surfacing.
last water change was months ago.
my tank goes against common practices in the hobby. and it's successful.
everyone's tanks are different, ... and for the most part, everyone experiences success to one degree or another, so what works, ... well what works is more than we think, and far more than we practice.
personally, if your getting readings 4-8, and your fish are healthy, your readings are wrong, time for a different test kit, and/or different lighting ... or ditch the test kit all-together, your fish are healthy, who cares what the test kit says, ... if your fish are not healthy, pull out the test kit, ... but an accurate one so you can figure things out and work on a solution.