Diego if you wany heavy you need wattage. Theres no substitiue. Heres why:
For us players, watts do not equal volume. For example you can get 100db at 1m with 1w.
Watts are a god indicator of headroom - and by headroom i mean bass. Bass frequencies require more wattage to reproduce because the speaker needs to move further. As a result the amp needs more power to push it. With insufficient wattage, you simply cannot produce enough power to reproduce enough bass. El84s, when used in pairs or fours can produce 20w or even up to 50w. 5 watts is simply not enough.
Now even tho you hear about all the heavy players talking about tight bass, they are still using big wattage amps like 100w marshalls or big mesas. Sure they tighten things up with a tube screamer to knock off some lower end, but there is still plenty there. And the amps (and speakers ) can reproduce it with ease. Thats how you get tight - by not maxxing out the power tubes. Those super heavy modern tones you hear on the radio are mostly preamp crunch with the power tubes just running warm, not totally saturated.
So - will a lionheart do it? No.
Will a blackstar 20w -well its no JCM marshall or dual rectifier - but yes its a great little amp that will give you some pretty great (heavier) tones.
Despite what all the ppl who play guitar in their bedroom believe - you still need wattage to get bottom end in a band setting.
Out of the two amps you mentioned, then the blackstar is better. But if you gonna play it in a band, then maybe look around for a 50w jcm900 cos they are still pretty cheap - and you can run the power section at 25watts if you want. It will get fatter with more volume, but you will be able to get some killer heavy tones at low volume too - especially if you like your pedals. EL84s juts do not offer what an el34 can do. Yes they can sound great and yes they can do some great crunch sounds - but they physically cannot reproduce the same frequencies the same way. Most 900s came with celestion g12t75s, which is a more bottom heavy speaker, They do not have a lot of top end so they dont icepick your ears - they just seem to get fatter and ballsier as you turn them up.
Dont fall into the trap of thinking low wattage = manageable volume. Its not that simple. And youll find a lot of ppl turning up their little amps because they cant hear or feel the bass.
The main determinant of percieved volume is speaker efficiency and spl combined with the resonant peak of the speaker because some frequencies are detected by our ears better than others. No you wont go deaf with a 50w amp necessarily, you can go deaf with a 1watt amp! Its all about frequency response and reproduction.
