It's a good reference
For any future purchases. My curiosity is getting the best of me and I'm dying to see the trade in pile at the local dealer's office 
The issue I face is justifying the cost of another D4. I paid all of $20 for the SE and the most decent looking Performance Edition for sale locally is astronomically overpriced for a 30 year old vacuum. I've noticed that out of all their models, the D4 was the most well loved and as a result they're prone to issues, unless you get lucky. Maybe I'll go for a nice E2 Platinum... I loved mine and would be willing to pay $100 for a complete one 
At any rate, my faithful D3C is still going strong and runs as happy as can be. Haven't done any major work on it since I bought it 6 years ago, the most I did was swap out the spider and somehow it's more powerful than the SE ever was?
I have so many good cleaners to choose from but sometimes there's one machine you just can't get over 