I was going to suggest using EQ to extend the lows, but I am not certain that is where your concern is.
The first bass rig I bought was an MTI SVT and 810. The 810 took low frequency EQ very well, but no amount of corrective EQ could get the cab to produce the sweet highs I like.
The B810 driver rolls off sharply at 5kHz, so I think the problem is not purely with the driver. Rather with a 410 configuration, I believe you get a lot of off axis cancellation that reduces the perceived HF extension of the cab. If you were to mic one of the drivers, I think you would hear more highs than you do listening to the cab in the room.
If you want to extend the HF response beyond 5kHz you will need to use a mid driver or tweeter. I think typically a horn is brought in around 3.5hKz with just an HPF, and the woofers are run full range.
If you want clear and articulate mids and highs, you probably need a mid driver and a full crossover so you don't get the off axis comb filtering in the mids from 410 configuration.
My solution was to biamp and run a Fender Twin loaded with JBL E120s for the high range. I ran an active crossover at 200hz so the 810 just handled the lowest frequencies, and most of the tone came from the Fender and JBLs.
Another way to discuss what is going on with the off axis comb filtering is to describe the formation of primary and secondary dispersion lobes. A primary lobe will form and shoot straight out from the center of the array. At various angles off-axis the sound fronts have different phases relationships. So at some angles the sound comes together in phase, sums, and produces a secondary node. At other angles, the sound comes together out of phase, cancels out, and produces a null. The number and angle of the secondary lobes depends on the frequency and the spacing between the drivers.
I don't think this graphic is 100% accurate, but hopefully you get the idea.
If you want to understand this better, take try to understand what is going in the attached Bass Array Doc. I also included a Frequency Wavelength table.
The bass array document is specific to subwoofer arrays, but the same exact patterns of lobes will develop at higher frequencies. The emergence of the pattern is dependent on the relative relationship between the wavelength and driver spacing.
For example these images are from the Bass Array document. Note that dispersion pattern in Fig. 42 and Fig. 43 look the same. However Fig. 42 shows 13.1' driver spacing and 100hz (11.25') wavelength) and fig. 43 shows 26.2' driver spacing and 50hz (22.97' wavelength).
The point here is the spacing and wavelength of Fig. 43 is intended to be exactly half of the spacing and wavelength of Fig. 42. Since the relationship between driver spacing and wavelength are maintained, pretty much the same exact lobing pattern emerges, just at a different frequency. Hopefully you see how this would relate to the formation of the some lobe pattern with your 410, only at a higher frequency due to the closer driver spacing.
Another problem that occurs is the individual cone drivers tend to get beamy towards the top end of their frequency response. So even though the B810 has response to 5K, the dispersion pattern at 5K may be a tight cone shaped beam that you will not hear unless your ears are within the relatively narrow dispersion pattern.
If you have the cab flat on the floor and stand relatively close to it, you tend to hear none of the highs, but you hear all of the lows because they are omnidirectional. I believe you will also tend to hear a peaked frequency range in the low mids. I believe this low mid emphasis is a secondary node aimed up at angle towards your ears.
Try elevating the cab and tilting it to aim at your ears. Another solution is to simply stand far enough away from the cab so your ears are within the dispersion pattern for all frequencies. In other words put your ears in the primary node.
I have a 215 Dually and need to be stand about 12-15' in front of it to hear all of the highs.
