It seems you already have the budget for AK240. If that's the case, I would not recommend 901 over AK240 mainly because AK240 is a more recent model with better UI and features.AK240 will have a better resell value considering 901 is not a popular product. The 901 gets a bit warm when using balance card. I find Hugo sounds better than AK240 but it is a portable DAC/AMP. It is not so ideal for walk around listening so it depends on your listening habits (whether 90% of your portable listening is stationary). If you sell your AK120 and can live without one microsd card slot, you can also consider the AK100II and AK120II. This will give you an upgrade with money left to spend on better things (maybe buy your wife a gift, too).
Well that is true until AK puts out another players...as they do very 3 months...look how many AK120 are unsold on the FS thread. More than 901s. I think the best thing is to try them with your iems as synergy is key.