I've been a largely happy RH (450 then 750) user since the 450 first came out.
It's an amp that has powerful and sensitive EQ and was designed with specific tone and response goals. I think separates many users and potential users into distinct groups…and there are folks who like it and legitimately dislike it. People who like it include those who find their sound out of the box and those who find their sound, but only after harnessing the semi-parametric EQ. People who don't like it include those who don't like the sound out of the box and never attempt to get into the EQ capabilities (or don't have success with it) and those who simply don't care for various characteristics of the designed sound.
If you value simplicity in EQ, the amp can be hit-or-miss; you can do basic EQ, but if that doesn't get you there, it's going to be a miss. But it sounds like you're looking to put the time in to set up your sound.
In my use, both the sound and the features keep me coming back. The features are things that might seem simple enough to replace or do without, but I've found that having and getting used to them makes them far less than easy to give up or do without. The tuner is fantastic and simply saying "It has a tuner" doesn't come close to capturing the value and convenience over time.
To your concerns
1. As was already said, strictly speaking, you don't "need" to replace anything, but I'll give you some opinions as to what I'd do and the reasoning. I take it from the (nominal) power handling specs that the Behringer cabinet is a 410? And from that, I'd make the assumption that it stays in the rehearsal space because of size/weight/not being needed for gigs (but rehearsals can get a little loud)?
What I'd consider doing (given the assumptions I'm making about your overall situation) is replacing both the Behringer and Markbass with a lightweight 212 cabinet. Generally, the idea of replacing both goes to your goal of having more consistency across multiple scenarios. The cabinet plays a significant role in the sound you get…using two separate cabinets doesn't provide any more consistency than you already have. That's to say I wouldn't see storing separate presets in the RH750 for two cabinets as being effectively different from using two separate heads with those cabinets (in terms of consistency). I'd look at 212s because, as a general matter, they present good compromises in terms of cost/weight/size/loudness between 410s and (in your case) a 112. You should be able to find options that are a reasonable carry relative to the Markbass and that can handle replacing the 410 for rehearsals.
If I were going forward with a new head to use in these multiple scenarios, I'd see replacing the Markbass as a given (whether letting go of the Behringer or not). To me, it just doesn't make sense to use a combo as a cabinet. I'd want to let someone else use it as a combo and put the value that added into a better cabinet.
2. Announcements come when they come. Nobody here can tell you exactly when the RH750 will be revamped or replaced. But personally, I think it's more likely than not that it will be revised/replaced in the relatively near future. When that happens, the price will be dropped, and they'll blow them out until they're gone. That said, the price and value today is what it is today. If you can't handle or accept the idea or potential reality of buying something today and seeing the price cut in half in a week, a month, 6 months, probably best not to do it. In the marketplace we have, I don't think it makes sense to speculate that a model will go unchanged indefinitely, and we're past 3 years now. There are lots of options…get a newer model.
3. Regarding headphone practice and the idea consistent sound you're looking for, I'd just keep using the existing setup. Again, the cabinet plays a significant role in the sound. I've used heads for headphone practice, and I don't think you'd be gaining anything…it's more cumbersome to set up.
Also, being new to this forum, you're undoubtedly unaware that the RH450/750 have a history of having generated a good bit fairly intense/heated discussion and debate. The fallout has been that there were and continue to be a fairly vocal group of detractors (even now, any post on the subject of the RH is likely to get some of that fallout). Their belief seems to be that the RHs represent an egregiously dishonest marketing ploy.
The basis of this issue was an article on the RH450 by Bass Gear Magazine. If the issue interests you, I believe it's a good idea to find and read that article for yourself to form your own opinion. The Cliff notes from my reading are that the RH heads defy conventional RMS wattage measurement and rating as used by consumers to gauge how loud amps are expected to be (i.e., with this amp, RMS wattage is not a relative gauge of how loud it is). The author concluded that this is not a trick or a rehash but is inventive engineering that delivers on its promises.
Bottom line on that, you've listened to it. It has features that you like. If it sounds good (to you), it is good.
. Do an extensive research first before getting the RH750. No amp is perfect. Could be a matter of luck as well. Same goes with other brands.
