I also have an outside system though it is no where as high end as my primary one. During the spring - fall months I listen more often to the outside one. Sounds great outdoors! It is also used indoors and placed outdoors for listening sessions.
I believe high end is about the approach one takes and I used the same approach in developing both systems. Balance the budget across all quality components.
I agree with Tonykay that there are many dated (~10 years old) quality HT receivers on the used market that can make for a reasonable 2-channel system for your purpose. They usually sell for ~25% of original retail. I believe this approach to be better than the purchase of a cheap mass market receiver.
I take it from your OP you are interested in FM and wish to compliment the PSBs in a way that the Krell does. If that is true a better approach might be to out a used 5-10 year old 2-channel receiver. They are likely to provide better FM reception and higher sound quality.
Regardless of the HT or 2-C approach, models from Yamaha, Rotel, Dennon, Onkyo would qualify. There are others, search Ebay and AG for specific make/model then web for user thoughts.
I am not familiar with your PSBs (however have heard many other PSBs and have enjoyed them).
How important is FM?
What other source will you use?
What is your budget range?
Do you really need 100+ watts?
Sorry, it seems I have rambled and not offered much advice. I am however interested in your quest as I believe a quality outdoor system can be had on a budget. Would be glad to exchange more info with you in it.