I wouldn't suggest the marantz. It sounds like you're interested in pushing your components to their limit and a surround receiver just doesn't make sense in a 2 channel application.
You have to first decide how you want your source to work; streaming or file playback. It sounds like you stream music so I might recommend a bluesound node 2i (made in canada) which I've recently purchased and has turned out to be an excellent source. The blueOS app works very well and you can connect multiple streaming accounts to it and control everything through your phone. The bluesound is much better sounding when the ethernet cable is plugged in, so take that into consideration. One advantage of the bluesound is there is an internal DAC that is fairly good (burr brown chip) which you could use before purchasing an external DAC.
I personally would go the route of an integrated amplifier without a built in DAC. DAC technology is changing at a very fast pace. This strategy would let you upgrade down the line. Keep in mind, the bluesound does not have USB out, so if you are interested in DSD, you would need USB streamer.
For an amp, I think you could be fairly flexible. I would choose an integrated 2 channel amp with one or two sub outputs. It doesn't sound like a phono input is desired so you could find units without that option to save money. I think bass management could be done manually if you get the right sub. Two options are the rythmic f-12 and the svs sb-3000. Both have integrated bass management that can be manually done; the svs within thier app and the rythmik on the back of the amp. One problem with the krell is that it doesn't look like it has a sub output. Depending on your goals that could be a problem. The bluesound node 2i does have a sub-out, however its fixed volume if you use it properly which isn't ideal but can be made to work. You just set your sub volume to a typical listening level and then manually adjust it beyond that.
One option is the musical fidelity m3si or m5si. Both don't have sub outputs but it looks like there are good deals on units right now if you do some searching. These are well respected amps however they have phono inputs and no sub output. Another option would be a peachtree nova 150. This unit has a built in DAC and phono but has a sub out. That is a class D unit which it sounds like you want to stay away from. Honestly, for budget, you could spend 2k on an amp if you wanted to, it would likely sound really nice if it pairs well with your speakers. I might try to get input from others who have experience with the Sierra 2's as I do not.
Also, don't forget to budget for cables and stands! They really do matter.
There are a lot of things to think about when building a system, its exciting to be starting out! Good luck!