Last night at band practice I dialed in a what we thought was a good bass tone with the b600h and 410. However, I still felt like I needed more CLEAN headroom. In this band we always use drop tuning and I need a very solid bottom end. I feel like I also need better note definition than this rig is currently giving.
This is why I'm trying to figure out what my best option is: more speakers (another b410), better speakers, or a higher quality 4ohm 410 such as the avitar.
Adding a second 8 Ohm b410 will net you around 6 dB of volume. I think what you are actually running into isnt lack of head room from your amp, but reaching the mechanical limits of your cab due to the amount of lows you have dialed in. In that case adding a second cab will net you more volume, but it still may not be what you are looking for.
Your head is decent, I have one in a similar vein (mark VIII). I might look into a different cab if I were you, but it wouldnt necessarily be an Avatar to begin with.