Headroom describes the effect you get at higher volumes with an amplifier - so my WT330 is rated at 330 watts into 4ohms but actually it could handle a transient of double that amount - hence the 3db headroom without being 'stressed'. If I were a dishonest manufacturer I might claim my WT330 is rated at 660 watts into 4 ohms but to attempt that would be distorted and dishonest because you could never achieve that result in practical use. However my WTX260 is a Class D amplifier and will give me (I'm in the UK at 230 volts) 290 watts into 4ohms but no more than that - it is a digital head so the amplification is generated by a digital process and cannot exceed - it would just stop working (never happened yet) when it reaches that maximum figure. As a result you might say that in analogue terms, the terms of the WT330, it would be more honest to claim a maximum of 145 watts... Now this is a topic we have discussed before and most people (I think I'm right in saying) would say that the Class D amplifier should really be described in those terms. Objectively there is very little difference, it would seem, between an amplifier putting out 290 watts and one putting out 330 but as I use both on a regular basis I certainly would accept that the power handling ability of the WT330 is far in excess of the WTX260. The WTX260 is light, it's a lot less to cart around particularly if I gig in the centre of town and have to walk extended distances with my gear, I love it and I use it far more than the WT330 for gigging - but when I get the excuse to use the power of WT330 and both of my EX112s I really enjoy it. I'm no technician which is why I have explained this in terms I understand which might not be wholly technically accurate. After all an atom is for most of us the smallest particle, as we leaned in school physics and chemistry lessons, even though in particle physics it can be broken down further and further and when you get into the quantum world even stranger things occur.