Timebase and MIDI

Hi, I'm new to the forums and pretty new to Reaper and DAW's. I used to be a tracker guy (mainly FT2) in the 90's, but when I decided to get back into making music early this year I was absolutely blown away by the available software, and especially impressed with Reaper. Anyway...I can't seem to figure out timebase. I've searched the forums and they've helped, but I'm not quite there yet. For example, last night I recorded an audio bass part without a click track. Then I added MIDI drums, matching the imperfect tempo of the bass. Then I tried to tempo map the project so that the grid would line up with the actual song and make it easier to edit drums on the piano roll. But as soon as I changed the project tempo or inserted tempo markers the MIDI drums went out of time with the bass, even if I set the drum timebase to "time". Actually, I tried setting the drum timebase to each of the 3 options and it didn't seem to change the way they acted when I changed project tempo. Is there some way to make those MIDI notes stay in place (with respect to time) when I change the project tempo? I'd like the MIDI item to behave like the audio item, and I thought setting timebase to "time" would do it, but this just seems to demonstrate that I don't understand timebase!

timebass 

Hey Matt, there is a "Lock Item" option if you right click on the midi track in the project view. I wonder if that will help you maintain two different tempos on one project.

Thanks Axiom. Actually I tried that too, but it didn't seem to have any effect. The drums still moved around on the timeline when I changed tempo.

Thank you Gofer. I just re-recorded the drums after matching the grid to the audio. Since there are several tempo changes in this project I didn't think the "Ignore project tempo information, use:" option would work. So, if MIDI items are always beat-based, why is there a menu option for changing the base to time? Does it change the item's behavior in some way? You mentioned MIDI is beat-based by definition. Do you mean the MIDI standard is beat-based? So I shouldn't expect any DAW to handle MIDI items as time-based?