I've insight into both HP and IBM worlds and can state the following:
MSA and EVA that often bundled with HP servers but are heavily sold into HP server accounts. EVA has LESS independence than the XP. XP is the best platform in the market and as such is successful in accounts where there isn't necessarily an HP server foothold.
XIV not as popular as you suggest in non-IBM environments. A number have essentially been given away to winback customers who are still figuring out in some cases what to do with them.
DS8000 only purchased by IBM mainframe / system p customers. Not a strong enough proposition on its' own, only stacks up when bundled with high end servers with the offer of additional discount off the server.
EMC until recently have shunned the channel so Dell actually have picked up a lot of mid-market business that has gone under EMCs radar, whether the customer is a Dell server customer or not. Resellers will work with Dell as opposed to working with EMC as it is the same kit but EMC are not interested in the channel. Plus a lot of customers still have a degree of 'anti-EMC' sentiment but recognise that they have good products.