ATC SCM 20ASL Pro II Nov 2023 I purchased a pair of the ATC 20's, which are their current model. Like my Event Opal's, which they replaced, they are Active Studio monitors, but unlike the Ported Opals, they are a sealed box design. They use ATC's premium quality SL drivers (Super Linear). Also like the Opals, everything is made in house, from drivers to Amplifiers. They use discrete MOSFET class AB Amps, 200w for midbass and 50w for tweeter (continuous). Analogue filters used, so no DSP. For the Eagle eyed, I have added a pair of High level output sockets, visible at the top right of the control area. These are attached to the mid/bass amplifiers output and can be likened to a pair of speaker binding posts on a conventional power amplifier. They allow a connection to a sub woofer, which ATC have not thought to include. My Subs have 100kohm input impedance, so their attachment will not effect the performance of the ATC's in any way. I modified the rear of the Banana sockets I installed, adding the end from a bullet connector to each. This allows my cables, like the other factory fitments, to be unclipped if the speakers need to be dismantled for servicing. There aren't many pictures around of the internals of the ATC 20's so I took a couple of shots while I had things apart. The Linear Power Supply has a beefy Transformer. The ATC's sit on top of a pair of BK Electronics XLS 300 FF Subs. They work very well together and are remarkably similar in design. Both are sealed boxes, use all analogue filters, discrete class AB amps and chunky Linear Power Supplies. I'll be honest, the ATC's are very underwhelming on their own, with an f6 of 55Hz, they really need the subs for normal listening enjoyment. With the subs, they are superb, all the top end finesse and mid range detail that these ATC's are famous for, but now with decent grunt and slam when needed. As stand alone speakers, the Event Opal's beat the ATC's, due to superior bass, but with subs added to each, the ATC's nose ahead. This active speaker stack has real synergy with my Pas Labs Pre and source equipment. Tonally I love the combination, which is very realistic and can sound "in the room" at times.