Ahh, I should have clarified ... I don't have a Tristar and therefore no experience with it ... but in terms of the Brain Bag only, it may not fit! I had the same issue on JetBlue. The BB wasn't fully packed, but it fit easily under one seat and had to be crammed under another - the variations are hard to accommodate if the BB is full!
Does this mean I should get a Tristar so I can be more helpful on the forums? Hmm ... ;)