How do instruments and vocals sound split between HD15 and mid horns above?

HD15 sound great for live music.  Lots of people complain about the upper bass from the HD15's sounding boomy, honky, etc.  Two things are absolutely need for the HD15's to sound good.   First proper wadding in the chamber and most important out of band EQ.  The HD's have a huge peak around 160-180.  This has to be taken down even if crossed over at 100.   I cross mine at 150 and they sound fine as long as the above mentioned hump is removed pre-crossover.

I've not heard of this "out of band EQ" technique before - any reading materials you could recommend please?

For heavy sub bass live material x1's are fine but need to have as many as hd's.Also your source material (kick drum/bass) must be capable to reach down there 45hz-85hz or the woofers in the x1's wont move at all.

I've used this combination the previous years (4 hd's + 4 x1's per side) for countless live gigs and it was more than nice.It took some time to come to a pleasant tuning on the system depending on the place but in the end it was worth it.

Basically, if there's a peak in the response of a box that's even a bit outside of the frequency range you want it to play in, it can still affect the sound. That's because (most) crossovers aren't brick wall type cutoffs but rather roll off at a more gradual rate.

Using a band of EQ outside of your crossover point to cut such a peak is a trick to help such a box both sound better and make it easier to get a good time/phase alignment through the crossover region.

Here's an example from the design notes for 18Sound's dual 21" manifolded sub to hopefully illustrate what I mean:

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Steeper filters have more phase shift, so take more care to get the phase/timing right between adjacent passbands, in extreme cases they can even add audible ringing to the sound. A moderate filter like LR24 + an EQ that only affects the problem frequencies can help avoid those complications.