In order to help understand why the distortion/overdrive pedal makes little difference to your dirty channel and makes a world of difference to your clean channel you need to consider where in the signal path the distortion is occuring. It will be very difficult to achieve the sound of a dual/triple/whatever valve rectifier by distorting and/or increasing the input signal and running it through the dirty channel of your DSL.
Generally your existing high-gain channel will be shaping the tone a LOT, probably close to brick-wall processing. Especially at high gain settings you will tend to notice any signal modulation before the amp less and less. The ultra high gain amplifiers that you're looking to replicate have many gain stages to build-up the levels and tones of distortion. By putting a booster/overdrive box infront of the preamp stage will boost the signal into the first stage in your dirty channel and provide more distortion, but if it saturates this first stage there is nothing more that the amp can do with the altered signal. It will just take this much dirtier clipped signal and do the same processing to it in the remaining stages, resulting in a very similar sound to before. (note: this a very crude approximation, it's not the place for amplifier theory)
What needs to be realised is manufacturers of pedals have been making all these high gain distortion boxes for exactly this reason. They are to roughly emulate the style of high gain amp stages, to be played into a clean or moderately dirty channel. The trouble again is they very heavily shape the sound, but it's the only way to do it. If you don't shape the sound the result is just a mess of unwanted harmonics and unpleasant distorted frequencies. That is why there is such a multitude of different pedals out there, even by the same manufacturer (ie Boss).
Alternatively you can do what some people here are suggesting and buy 2 different boosters/preamps where one will drive the other into distortion, pretty much what is done in a distortion box anyway but essentially it adds a couple of very controllable gain stages infront of amplifier, giving you a little more scope.
As to the question of which pedal? I can't give a recommendation. Majority of guitarist will buy and sell pedals very regularly in search for the holy tone or on other peoples recommendations. Remember it'll never sound like it does on the record anyway, you need an entire production suite to do that :)
i hope that helps a bit in the understanding of it all. There is no magic answer :)
regards,
Tom Johnson
Legra Guitars
