It sounds counter-intuitive, but, my advice is to not concentrate on the lows or sub frequencies any lower than a good B (i.e. don't boost 30hz willy-nilly). In fact, on stage, it clears the mix up if they aren't there (or are carefully controlled if they need to be). Yes, it is true that most on-stage speaker systems can't handle the fundamentals anyway, so why put them through the stress of trying.
The funny thing about frequencies is what we believe to be there when they are not - and to take advantage of that. Case in point, We have all enjoyed monsterously chunky guitar riffs that sound deeper than satan's codpiece - but, when solo'd in the mix, the low end is often shaved off 100hz or higher purposely.
For us bassists tuning low (I've tuned to E0, my 7 string is currently tuned to F#) , I have had no issues with using standard backline taking in to consideration the above. TL/DR:- Know that there is more weight - or 'girth' as folk like to call it in the 80-160hz region for bass guitar - for obvious reasons and that's where I start carving my bass tone. You don't need a 24dB/Oct High Pass Filter on the lows if you are careful, but it can help. Start at 30hz and work up until the beef tightens up a bit. Next, there's a balance between what your bass needs to sound full and what the mix dictates. I wouldn't want a wimpy bass tone, but, we can't all fight for the same area of the frequency spectrum in the mix (and this is also where the decision is made about who gets the the higher crossover point, or sitting forward in the mix - the guitarists shaving away their lows to make way for a fatter bass tone, or, the guitars taking the lion's share and the bass is relegated to unison lines and a more underpinning role. Think Karnivool versus Alterbridge as examples.
Back to bass again - and, remember the mids! I'd use band-pass distortion to add energy and life to the midrange to cut through where needed and, as I have said a million times before, I am a huge fan of parallel compression, more specifically upward compression to create a big 'bed' of sound that kinda acts like the smoothing compressive qualities of distorted guitars. (to help them blend) - If you have access to Parallax by Neural DSP, it uses a well known studio method for bass that entails compressing the low frequencies and treating the higher (driven) frequencies separately. Coincidentally a video from either Produce Like A Pro (Warren Huart) or one of the online academies came up on my feed again discussing this very method recently. - You can either go in hard on the QC with this or leave it to front of house.
Funnily, even though Billy Sheehan doesn't tune low, his method of splitting the bass signal in to 3 isn't far off this idea, but many other bassists have split their signal between different amps for example to straddle the low end support and still being able to be heard in the mix. Duff McKagan has recorded using both a bass amp and plugging in to a guitar amp for the mid range and top end bite. Dug Pinnick's tone is another option. There's lots of low end in there, but his more aggressive top end (easily attainable with the Tech21 DP pedal) will help your pitch clarity come through. I'm also thinking about awesome bands like Vola or Gojira for their bass tones here too.
Anyway, as you have one, the great thing about the QC is that you can do this virtually with almost limitless amp combinations!
Finally, I suppose another /TLDR is that a down-tuned instrument sounds down-tuned. Doesn't it? Drop D on a bass just has a different tone. It still sounds drop tuned on an iPhone speaker, and given the amount of people who can't tell there's nothing below 100hz coming out of one of those...
/sorry if this is a ramble, I'm half asleep still..