I agree with sonicsmith, I think the 75 is in a sweet spot for ARC, who seem to produce the great, classic amps in the 50 to 100watt output window, like the D60 for example. I certainly preferred the sweetness you describe, to the sound of the Ref 150.
In answer to your question, I don't think you lose anything with the SE upgrade, just gain better grip and base in particular. My amp is staying till it gives up the ghost and if I have to change speakers, I will make sure they match the ARc's output, but my Daedalus speakers ain't going anywhere either.
If you want to get the flavour of the SE upgrade, you can by just changing the tubes, it's quite safe, I and many others did it and you save on ARC's tube prices, if you buy from say Upscale audio. The only problem is you can't put the top of the amp back on, but no problem. It will give you a large part of the SE upgrade, without the cost. ARC may huff and puff about using the tubes in the factory upgrade. They told me they might not be perfectly matched!! Silly me, I thought that's what adjustable bias was for. With the full upgrade, the amp top can fit back on.
In truth, I am sure I would be very happy with the Ref 75 and 150. Both I expect, will be revered amps for years to come and will maintain a good resale value.