I need a new amp that covers the following:

1) Ideally a combo that I can hook up to a 4X12 for gigs.
2) Versatile as I play in a Rock covers band so use a range of tones. 80s Metal for sure.
3)MUST be able to take a boost for solos. My DSL401 doesn't on the drive channels.
4) Valve

Contenders so far:

Marshall DSL
Good tone but only a head unit and latency on channel switching a pain.

Marshall JVM
Veratile but not cheap.

Laney VH100R
Seems to have potential but read mixed reviews and a head unit rather than combo.

Engl Thunder
Sounds good. Layout looks similar to my DSL so I'm concerned it won't take a boost.

Hughes and Kettner Switchblade
In many ways seems ideal. Can't try this first as nothing local. Some clips online sound a bit harsh and modern.

Any thoughts anyone?
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PRS SE 'Floyd' Custom 24 with Creamery pickups
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The Laney VH100R has amazing cleans and amazing high gain tones. Just in between that it is a bit lacking. So I'd say it wouldn't be great for rock.

Marshall DSL's are a bit meh as far as tone goes. There are much better amps for similar prices.

I can recommend that ENGL though. It is a great amp. It should take a boost just fine too.
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My DSL401 takes a TS9 just fine. That, along with good tubes (JJ el84s and JJ/Tung'Sol new production 12ax7s) and a WGS Veteran 30 speaker really brought mine to life for metal playing, and might do what you want of it just fine if you intend to keep it anyway.

That said, I'd like to add to this list the Orange TH30, and strongly recommend it. It is what I'm saving for at the moment with similar desires. I primarily use an 80's metal tone, and it is quite capable of that, but my experience in store with it is that it really is as capable as or more capable than the many youtube reviews show it to be. There is a head and a combo version, and seeing as you want combo + ability to plug into a 412 it seems volume is a concern, and the TH30 has half and quarter power modes down to 7.5W. 30W is more than enough through a 412, especially mic'ed, for stage use.

From your list, however, I would pick the JVM, for all the reasons above (aside from the half/quarter power modes, but you can turn it down and it holds tone better than most tube amps being preamp-based high gain design). Yes, it's expensive, and obviously that may remove it from the running for you, but if you can keep the DSL401 going with the above advice, even if it costs you a bit in the meantime, you could save for the JVM.

You WILL NOT regret either the TH30 or the JVM, but I cannot say enough how wonderful the Orange is - it's relatively new and word hasn't spread yet, but I guarantee it will!

TL;DR - play an Orange TH30 if you can, or save for the JVM, screw the other stuff.
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Orange Micro Terror + cab
Digitech Bad Monkey
Zoom G2G
It's strange. My DSL has a Bad Monkey in front of the amp and an MXR Boost pedal in the loop and neither makes a significant difference to the volume when on the drive channel.

The more I think about it, the more I think the versatility of a combo makes sense and it only has to be a 1X12 as I have the 4X12 to fall back on anyway. That leaves the possibility of a JVM, the Engl Thunder and the Switchblade. The JVM combo will be upwards of £600 and there are none on Ebay right now though they come up regularly. The Engl Thunder doesn't come with a footcontroller and I suspect I may find the increased versatility of the Screamer a better option. That leaves the Switchblade which seems to divide opinion and the footcontroller is rather fragile but it does appeal. I'll take a look at the Orange amps too.
Gibson Les Paul Studio with Catswhiskers pickups
PRS SE 'Floyd' Custom 24 with Creamery pickups
Fender Standard Stratocaster with DiMarzio pickups
Takamine GN30
BluGuitar AMP1