If it has to be a Nailbomb I'd also be inclined to go alnico because of the better leads and greater versatility but I confess I'm still hesitant as it really is an 'all or nothing' pickup. It's also, to my mind at least, nowhere even remotely close to the 'middle ground' you're talking about if one of those reference points is a Holydiver. As a result of all this, perhaps it's time that we review what we have and perhaps look at some 'left field' solutions. Everything so far has come from the Contemporary section and you've dismissed the Juggernaut, Holydiver, Painkiller, Black Hawk and Aftermath already. The Warpig and Miracle Man will have too much low end so they can go too, as can the Crawler which won't be tight enough anyway. I have severe doubts the Impulse will be what you want and there is clearly doubt about the Nailbombs. That doesn't leave us with much but it does leave us with a couple of intriguing possibilities.
Rebel Yell - Based heavily on the Nailbomb but toned down a bit. Personally I'm not sure in your guitar but if it's as balanced as you say and you want more upper mids with a good lead tone, this might be a better option than the A-Bomb.
Cold Sweat - This is nothing like as bright as the EQ chart suggests in my opinion, though it is 'bright' overall. A lot of this is due to the fact that as well as a strong top end, it has some nice upper mids that are nothing like as harsh as you get on the Nailbomb. It's tight so will take lower tunings like a trooper. It's also a fantastic lead pickup. This could well be worth a look even though its natural home is a Les Paul (like a Nailbomb).
After this you've exhausted the Contemporary section so I started to think about other alternatives. There are a number of pickups that can be said to be broadly related to the Holydiver. The Miracle Man retains that smoothness but with it being a hotter wind, it has the darkness you don't want. Going the other way, you get to the Black Dog and once I started thinking about this one, it started to really make a LOT of sense. It has the throatiness that you think of with a Nailbomb but toned down somewhat. It gives quite broad mids like the Holydiver but with it being a lighter wind, tends to be a bit brighter overall so it should give you that bit more boost in the upper mids that you're after without becoming harsh in any way. It should also pair very well with a VHII in the neck and give you great leads, as well as being easily tight enough for lower tunings, hence it's a popular Metal pickup.
https://www.youtube.com/watch?v=CaqEET376-IThis strikes me as far more of the middle ground you're talking about than the Nailbomb. The vintage hot wind coupled with modern wire gives you a great versatile 'middle ground' pickup with awesome tone. I really think you'd be surprised with this one as I think it might be your best option. Can't think why I didn't think of it earlier!