Their preamps! At least the one in the WT400, the only Eden amp I've hung onto over the years (I still have a D115XL and D215XLT mainly because I can't find a local buyer). But I really like the sound of the 400 pre so I decided to use it in a higher powered rig for small clubs and restaurants. Why? Read on... The EA iamp600 and the SWR Bass 750 neither one put out near enough watts into 8 ohms. The Mesa Boogie 400+ is sorta the wrong sound with the Bergie 210 which is very power hungry. One channel of my Stewart 2.1 (400 watts into 8 ohms) won't get the job done, either. Mono-bridged with the 2.1 is 1350 watts. A bit too much, I say. The Mackie 1400i (which I happened to have in the garage collecting dust in an unused rack) puts out a cool 1K watts into 8 ohms when mono-bridged. Voila! The solution was to pair it with the sweet sounding WT400 pre. The only downside is: this rack is pretty heavy. But it sure sounds good.