Where to insert tubes while running a Luxman SS integrated

Hard to say without knowing the rest of your system. You may want to list your components in the Virtual Systems section of your profile or list it out on this thread. 

There are many better preamps than the preamp of your integrated, but the unit is no slouch. It uses Luxman’s famous LECUA 1000 volume control which is the same quality that was found in the better Luxman separates. Going with a new preamp means you’re giving up the other features of your amp, including the phono stage, but it also makes the sound more consistent between your vinyl and digital, if that is important to you. There’s no right or wrong way, but you’ll need to decide based on your priorities and tradeoffs you are most comfortable with. 

Hard to say without knowing the rest of your system.

this...

Thanks for your input. I’m currently running KEF R3s with a Pro-Ject X2 (Sumiko Moonstone cart) and a Bluesound Node 2i. However, I’d like to keep the Luxman and upgrade the speakers and sources. I’m thinking of moving over to the Tannoy legacy series, so any input based on that system would be appreciated. 

My journey only started a few years ago and have been gradually upgrading. The KEFs are nice compared to what I had before. Their imaging is probably my favorite aspect of them, but they can be harsh with poor recordings, which has become an annoyance. 

I would recommend you to get a DAC with tube output stage.

Either Lab12 DAC1 Reference or Aqua La Scala MKII. 
 

i understand the need now... luxman solid state into kef r3 will take no prisoners with poorly recorded music -- treble lives close to the bleeding edge

try to find one of these units on the used sites (the links are to already sold listings), they will preserve the musicality while dialing back the unwanted sizzle and bite

modwright modded cambridge cxn v2 tube output

mhdt tube buffered dac modded

any older model van alstine tubed dac on the used market (they are all excellent in performance and value)