Ok, here's the deal:
For tube mech mods you basically have 2 things decide on when it comes to what you're calling 'power' (which is actually just conductivity and voltage drop)...
1. Hybrid or not hybrid
2. Which metal/metals (copper, brass, SS, aluminum, platings, combo-metals, etc.)
General rule-of-thumb for metal conductivity: from more to less -
1. Silver
2. Copper
3. Brass and aluminum
4. Stainless steel
(It's not *actually* that simple or straight-forward, but it's a good place to start)
Would you be able to tell the difference between (say for example) an all copper, hybrid tube mech vs an all stainless steel non-hybrid tube mech? Yes, probably...not a TON of difference...but some. Would you be able to tell the difference between the same hybrid tube mech in copper vs brass? Honestly, no. At that level we're talking voltage drop differences in the 1-3 HUNDREDTHS OF A VOLT! Anyone who claims they can feel the difference there is not being honest, IMO.
As for dual-battery box mechs - you also have 2 choices: Series or parallel. That's WAY too long of a discussion for this thread - but do look it up - it's very good knowledge to have.
In my humble opinion - based on what you're saying you want here - I agree with the poster who said you should probably just go ahead and get a dual 18650 regulated box....one that goes to 150 watts or higher. That is best and safest way to have a lot of power at your disposal. If you're not an expert coil builder and don't yet have a firm grasp on ohms law and how resistance effects amps draw and wattage, the get a device that 'knows it for you'.
PS - regardless of which device you get - PLEASE never, never, NEVER skimp on the batteries. Do battery research and buy the absolute best ones you can (with good QC, battery chemistry, high amp ratings, etc). You can never go wrong with Sony VTC 4s or 5s.
Good luck!