The last few posts about the M5's having better bass than the IE8's for 200$ less is suprising, is this true or is it more equal? (assuming bass knob has been maxxed out on the IE8's) because if they were at least equal I'd still be fine to choose the IE8's because of their superior mids and highs etc.
And can anyone let me know about whether the isolation in the IE8's is truly as bad as I've read in some reviews? I'd like it to be at least usable when I'm playing the drums.
The thing that might help you understand why there is such a massive price disparity between two IEM's that are both reviewed quite well, is that the Atrio design is a much newer design with newer drivers when compared to the IE8, and that is one of the reasons why you are seeing similar quality for less money. Sennheiser is a very established brand, so they can essentially rest on their laurels in terms of an old design and charge a premium, but Atrio is also pretty established among professional musicians and not so much the consumer market.
I can tell you that having heard the MG7 Atrios that they do really have an amazing sub bass response, comparable on bass alone to the Radius DDM's (and better in almost every other way), which is another relatively new design, again for much less price than the IE8's. Certainly I'm sure the IE8's are not bad, but keep in mind that they are a flagship model and have been available for a long time by IEM standards.
On another note, the Atrios would almost certainly have better isolation since they have much thinner nozzles and support the use of biflange tips, hence they can go deeper in your ears. Again, Atrio is a professional musicians company, I would trust them to make good IEM's for drummers.