SET amplifier recommendation

This is a tough one.  With an SPL of 88dB a 300B is going to be a problem.  You will get 8 to 13w out of most 300B SETs.  Let's assume 10w, you will be getting 94.3dB of volume at 10' from the speaker at full power.  That is not a lot of headroom.

In a mid-size room (20 x 20 is definitely mid-size) you are going to want a bit more power.  

This website will help. 

http://myhometheater.homestead.com/splcalculator.html

Consider your setup and speaker placement.  I find 85dB to be an excellent reference volume and you want about 15dB in headroom to cover dynamics.  Thus, you want to be able to deliver ~100dB in volume.  

Assuming your speakers aren't in the corners of the room and are near the wall (2-4' away) and you listen about 10' from the speakers, you will want at least 30-40

There are not many SETs that deliver that kind of power unless they are parallel SETs which tend to be expensive.  I import Art Audio and have done a lot of studying of SET amps.  The list is not long that will deliver real power.   

If you are looking for a little more in terms of PRAT you could try a different tube.  That ARC uses a 6550 which is popular because it is inexpensive and gives you more traditional tube dynamics than a KTXX but is not in the same league as a SET.  

What you could look for is Push Pull using a different tube type.  

I can only think of one 300B Push Pull from KR and it is not in your price range.  Opera - Consonance make a 2A3 Push Pull at 30w called the 880A.  I believe it should be in your price range.  They no longer have a US Distributor and I have been in touch with them regarding representation.  If you are interested, let me know and I will press them.  

300Bs and other lower powered tubes vs. 805s - 845s - 211s - T100s have pretty significantly different sound profile. 

The 300B, 2A3, PX25 has this lush midrange that borders on addictive.  It is amazingly musical and is what most people think of when they here a SET.  

805s, 845s, 211s, etc... have a different sound profile.  It is less lush and more dimensional IMO. Delivers tighter bass and a less pronounced midrange even compared to and EL34.  As someone who has an 845 SET and 845 Push Pull and use less sensitive speakers (current pair in my system has an SPL of 84dB paired with Push Pull) I am a fan of its sound profile. To me, there is far more difference between tubes than there is going from Push Pull to SET using the same tube.  

The Line Magnetic you are talking about is an interesting amp in that it is 3 stage instead of 2 stage and there are 300B Driver Tubes to help deliver a more lush sound.  I did a ton of reading on it today for a customer or mine.  It is lovely out of the box but can be improved by rolling in just the right tubes. 

Also, high SPL speakers are a completely different animal.  They can sound great.  Klipsch are wonderful.  Depending on the model, some require being place in a corner.  The Cornwalls are great speakers, work in most rooms and are wonderful with a 300B.  Zu also makes some great speakers and Tekton, assuming you like larger units, has some great units.  

If you really love your 805s, I mentioned above.  You might want to look at a push pull with a different tube compliment.