i've never used ambient IEMs. with that said, i use IEMs mostly for hearing protection, so i don't want any stage sound getting in.
general rule i stick to is that if it's not in the IEM send, i won't hear it. many people say they want to use IEMs and only put their guitar in it. well at a certain level (pretty low), you won't hear anything else since the drivers are so close to your ears and the IEMs are acting like plugs too.
i have used several IEM models from $100 to $500. in my experience, i don't think the expensive ones offer too much more than the cheaper ones. but i'm approaching this as MONITORING, not a studio high-quality listening experience. i'm not there to bask in the glory of the sound. i use the SE215s as well. cheap, easily replaceable, and sounds good with a little EQ on your monitor send from the mixer. in general i'm boosting a bit of high end, and reducing low end so it doesn't bog down the mix. i also pull out some mid range frequencies, very slim Qs, whatever the mixer allows.
some people swear by custom molds, but i am completely satisfied by the SE215 with the rubber tips.
for the mix, like i said, if it's not in the send, you probably won't hear it. even drums, depending on stage volumes. if there are vocal mics on stage, many times those offer enough "ambience" as they are open mics letting in sound. i personally don't need or want to hear crowd noise or stage noise - that's the whole point of IEMs for me. add what you need though. for drums, you don't necessarily need a full mix. just one mic near it - even a 58 - that's mixed into IEMs only, and not sent to the mains.
keep that in mind - you can add mics that only go to IEMs and not mains.
for stage volume, if you need interactivity with your guitar for feedback, etc. then yes you need to have that speaker on the stage. if you want to "feel" air moving from a loud cab on stage, then add it. it's all up to you. i play places that don't want amps on stage, so that's not an option and i honestly don't miss it.
it all depends on your goals for IEMs. for me it's clarity. some restaurants i play, the crowd noise is so loud from clanging plates and talking 3 feet away from me, and the venue doesn't want ANY speaker on the stage - sound only through the ceiling speakers. so it's impossible to play without IEMs. some of my partners use headphones instead of IEMs and it works for them. but i like the fit and feel of IEMs.
what you can do for your IEM totally depends on your mixer. a standard analog board doesn't have too many options. the digital offerings like the x32 or Touchmix are preferred.
i always EQ my IEM sends from the mixer. no speaker sounds perfect flat, IEMs are speakers, so i EQ those as needed. i described it a bit earlier. the biggest problem i see is too much bass frequency in the IEM and then you "can't hear" anything. reduce bass. see how low you can get it to function. you'll be surprised at how much clearer the mix sounds immediately.
again, i approach it for monitoring, not a relaxing home listening experience. it's a tool to help me do my job. it doesn't sound bad by any means. it's just not the mid scooped "LOUD" button in the car sensation. and i don't want that at a gig because then i can't hear things!
