well, there are two ways you can go... speakers on desk or speakers on stand. speaker positioning is the number one factor of chosing a desk. you can always move your other gear around, but you cant buy a new desk every time you try to position your speakers properly.
if you want to have your speakers on your desk, it will need to be very deep. i would say at least 90cm, otherwise you wont have any chance of positioning your speakers well, although i think 110cm is probably ideal. i doubt you will find any desks of this depth at ikea (i couldn't). a desk width of 130cm is probably the minimum.
alternatively, you could just put them on stands (i use samson ms200s - quite cheap). if you plan on using stands, make sure your desk is either very narrow (<1m), or very wide (>1.5m) wide. likewise, make sure there is an overhang at the back. the reason i say this, is that speaker stands have a very large base (usually around 40cm square), so you do not want the legs of your desk getting in the way. you also don't want your comptuer monitor being in the way of your speaker signal (it causes comb filtering), so the overhang at the back allows you to tuck the base of your stands underneath the desk. this should put the face of your speakers roughly in line with your computer screen.
professional music-specific desks are quite shit in my oppinion. i had a look at all the popular ones (legend, argosy, etc.) and they were all very poorly designed. firstly, they usually have rackspace beneath the computer screen. this causes your computer screen to be quite high and will destroy your neck looking upwards all the time. likewise, most of them are about 70-80cm deep, but have speaker shelves. obviously you cannot use speaker stands with these desks... but at the same time, 70cm is far too close to be monitoring at. you have no room for movement in positioning your monitors and just have to put them where they tell you. fale and aids.
in terms of laying out your shit, i think having a tray for your midi keyboard is awesome... keeps it out of the way when you don't need it! other than that, i'll leave the rest up to you.
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