If you are wanting a softer bitterness, there are a handful of techniques you can untilize.
FWH-First Wort Hopping. Instead of adding your bittering hop into the beginning of your boil, add it to your kettle when you start your mash-out. It tempers the bitterness, can't really remember the chemistry behind it at the moment.
More late additions- and I don't necessarily mean 1lb of cascade at flameout, but if you more more of your hops to the 25-15min mark, you will increase flavor and decrease bitterness while still extracting enough IBUs.
Keep your BU:OG ratio in mind. the bittering unit to OG ratio is important, it is, as you imagine the ratio of IBUs to points of gravity in your post-boil OG. For IPA's, many have found that a ratio of 1-1.2 is a really great sweet spot for IPAs. (example, a 1.080 wort should have between 80 and 95ish IBU)
Counter the bitterness with a slightly higher mash temp or increased crystal malts. It you give yourself a slightly maltier IPA, it will help hide the hop bitterness which will give you that soft sweetness, light crystal malts can help a lot here though personally I love crystal 120 in my IPAs.
water chemistry. Using softer water will pull out the malty soft characteristics while water with more calcium and sulfate will emphasize bitterness.
Yeast- certain yeast like WLP001 and Wyeast1056 will emphasize bitterness and understate the malt character. If you want a softness to the IPA, consider a different yeast strain, maybe an English or London Ale strain.
If you do some searches on brewing Northeast Style IPAs, it should give you a good start since Northeast IPAs are all about the soft bitterness.