The important piece of info here is in the rest of the system. What are the amps & speakers, tap settings, available headroom, etc.

If there is a lot of headroom left on the amps, then yes, 70V subs would be an easy option (using the crossover in the subs).

I would not run a 70V speaker level input into a consumer subwoofer and try to use it's crossover. As stated above, it's a huge difference in voltage. You'll be more likely to get smoke coming from the sub than sound.

You could use a line level crossover integrated in a consumer sub, but you will never really have proper gain structure as you are still mixing voltages and risking overdriving the crossover input to then underdrive the amp input. You are also specifying a piece of gear while it may be a great option for what it is intended is in no way, shape, or form designed to run 12+ hours per day every day.

I don't think the issue is so much mixing 70V and direct connect in this system as it is mixing pro and consumer gear.

That being said, if you have headroom, Bogen makes a 70V in-ceiling sub (CSUB). It's an 8", though, so it's probably not going to give the low end you are looking for. Ditto the JBL Control 19CS or 19CST. They are more used for supplementing real small ceiling speakers. Take a look at the Control 300 series and now you are talking. A Control 312CS is a pro 12" architectural sub with an available backbox and it will get some air moving. There is an accessory transformer available (MTC300T150), but its a 150W transformer as these things do need some power. Unless the original designer was really planning ahead, it's not real likely that you have 150W plus of amplification headroom. At that point it's probably more cost effective not to use 70V subs.

If this was a start from scratch system design, I'd be pointing you towards DBX ZonePro rather than the Bogen piece. It will do what the Bogen is doing for you plus give you the extra processing that you may have not known you needed up front, but now do. See, method to the madness. ;)

It still might not be a bad idea, but a good bet to get the DSP processing of the ZonePro without the routing, mixing, pre-amps, control, etc. would be to look towards the DriveRack series. They are all good products. While they are not necessarily designed for it, the PA and PX work just fine in fixed installations. You just won't have the preset EQ curve for the install-oriented speakers. This will serve as your crossover, EQ, Delay, etc. They also have sub harmonic synthesizers to really boost the lows, auto RTA to give you a head start ringing out the room, etc.

Assuming the Bogen stays, take it's output and feed it to the DriveRack, take the high pass output of the Drive Rack and feed it back to the existing amplifier & speakers. Take the low-pass/sub output of the DriveRack and feed it to a new amplifier and subs. Again, I don't really care if it's 70V. For architectural stuff the Control 12CS is a nice piece (with backbox). An XLS-series amp pushes a lot of power for short money. If you want to go 70V then add the transformer and go up to like a CTs series amp. That won't really gain you much unless it's a really long speaker run or a lot of subs per channel.

If you are using box speakers and prefer powered subs, the JBL EON sub is a pretty decent pro sub in a configuration that may feel a little more familiar to you. Just feed it from the sub output of the DriveRack or you can use the line level loop through (not speaker level) you asked about above and put it in between the ouput of the bogen and input of the exist amp(s) like you are suggesting above. The difference is that this is a balanced pro line level loop-through.

The EON SUBG2 is a 15" 250W powered sub with loop-through and they recently released a big brother EON 518S 18" 500W powered sub with stereo line level loop-through. Either of this will fit into the system like you are used to but be a lot more appropriate to the job.