......
18 pages and no clear solution except ...go full range? Which I'm not doing.
Oh boy. I need a drink.
The heads on the market that allow bi-amping are the worst feature ever put on a head. Clearly a marketing call to sell more heads. Why is it a bad feature? Because most of the people who will use it are those that have inadequate or inappropriate cabs and not enough technical chops to pull it off.
Bi-amping works
if you use a sub designed as a sub. Horn loaded boxes (often referred to as bass bins) are common, but there are direct radiator subs as well.
Bi-amping also works
if you use a top that is designed as a dedicated top, not merely a full range cab you have laying around. Many tops are arrayed boxes, but in any case, it needs to be engineered as a top.
Bi-amping also works
when the crossover frequency is chosen both in regard to dispersion characteristics and the cutoff frequency of the top, not
just the cutoff frequency of the top. This requires making and reading polar coordinate reading with an SPL meter, or using guidelines created by the manufacturer.
In short, leave bi-amping to the PA guys unless you have background in live/pro audio. And in that case, those guys use rackmounted active crossovers and multiple or dual channel power amps.
Now, you seem to diss a full range cab as an unacceptable solution. Yet 99% of bass backline gear is that way. Seems to work well for them. Choose a cab that suits your playing style and sound goals, add the right head, dial in purposeful EQ settings, and you are good to go! It's more cost effective and easier to set up too! (For this discussion, a full range cab can also be those that naturally roll off at 5kHz, not just those that go up to 16 or 20kHz)
Now, you mentioned you only have $500 to work with. You are in a tough spot because you wasted all that money replacing woofers in an inadequate system. You might want to consider to stop throwing good money after bad. You are also in a tough spot because you only have 350 watts to work with. That's about the minimum threshold for live rock with a loud drummer. You really need to be at the 500 watt minimum to get a head that will deliver clean power with enough headroom and you're not clipping all over the place or having to drive it wide open all the time. What 410 do you have? If it's a good one like an Eden, SWR, I would use my $500 and get a better head. If your 410 is an entry level cab, then I suggest going on CL and looking at some used Peavey heads and cabs.
Hopefully all the information in this thread will help you realize that to bi-amp properly and garner any benefits to outweigh the associated costs and additional gear burdens, you need sufficient knowledge and more money to work with than you currently have. The Fish bassist and John Entwistle pulled it off because they have engineers and large wallets. And Criley can pull it off because he's Criley. For the rest of us mere mortals, use the KISS* principle and get a full range cab.
(*
Keep It Simple Stupid)