300B SET or EL34 SET

Both tubes (EL34 & 300B) have a pronounced midrange profile but IMO (based on the amps I sell from Art Audio) is that the EL34 has a pronounced mid emphasis and the high's are a bit rolled off compared to say, a KT88.  The 300B has a lush, almost addictive midrange with excellent high frequency extension.  

Bass response will never be as good as with a solid state amp but ultimate bass response will be driven by the quality of  the output transformers.  

89dB is low for a 300B.  Whether it is adequate very much depends on your room.  In a smallish room with speakers near the corners, yes.  In a larger room.  

If speakers were properly matched to the amp, I don't think there is much of a difference in terms of bass performance between and EL34 and 300B.  Both will be a bit flabby compared to SS, but neither is much better than the other.  The 300B is the better sounding tube overall IMO. 

What matters most in terms of bass reproduction is the transformers.  Great transformers will deliver great (for tubes) bass.  Crappy transformers will deliver flabby bass even by tube standards.  Given the massive power draws, transformers for SETs are critical.   

Regarding those specific amps - the 300B delivers a shockingly specific 8.2w while the EL34 delivers 12w.  

It is no secret that 1W of power is adequate to deliver music at normal listening levels.  You need headroom to hit peaks in volumes because music is dynamic and SPLs vary from moment to moment. Average SPLs for quiet listening might be 75dB but you will find that volumes go up dramatically.  Get out an SPL meter and you will be surprised to find that peak volume's approach 100dB (especially at low frequencies) all the time.  

Assuming normal conditions, sitting 8' from your speakers and they are 2-4' from the wall, 8.2w will give you a max output of 96.4 dB.  That is really low.  12w will give you 98.1dB.  Normal listening will be no problem but you will find that bass will get very flabby with dynamic peaks in volume.  

For music, I like to target 100dB in peak output when matching an amp in terms of minimum power output.  This assumes the amp is well suited to dealing with the speaker impedance.  Those speakers really need a minimum 20w amp.  

If you are curious, this is a useful link where you can do quick SPL calculations.  

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

Other option beyond Audion - 

You could consider a few other options.  There are other tubes in the 300B family that can deliver very similar mids with a bit more power.  300BXLS, 520B and the 320B XLS/842 are all very interesting tubes.   

Examples -
KR has a 300B XLS bases SET that is 20w.  The KR VA340 is phenomenal but it is $12,500.  I could talk to Eunice and see if we could do a Black Friday/Cyber Monday type deal but it wouldn't be more than 10 or 15%. 

KR Also offers a 40W Push Pull 300B which is $9000, the VA830.  Wonderful amp that will drive those speakers unbelievably well.  

Art Audio offers an 845 or 211 based SET which will get you to 18w.  The 845 or 211 has an amazing soundstage.  This is my favorite tube and I have a pair of 845 push-pull mono-blocks.  The soundstage is deep and wide with shockingly flat frequency extension.  It won't have the pronounced mid-range that you get from an EL34 and it is miles from the lush mid-range you would get with a 300B.  These tubes require massive amounts of current, and you want to only purchase from very trustworthy manufacturers.  On-sale, Art Audio's are ~$8500 as an integrated.  The 520B based SETs are 2x that.  

Line Magnetic has an 845IA SET which is 22w and I think the price is in the $4500 or $5000 price range.  People rave about this amp and it has a very appealing price point but again, 845 based tubes rather than 300B.  The tube compliment is pricey as they use 300Bs in the driver stage.  

Opera-Consonance makes a few amps that are interesting.  The  Reference5.5 MKII Integrated amplifier is 18w as a 300B Push Pull.  It is inexpensive....$2K though it is made and china and only comes with a 1 year parts warranty, no labor.  Ships with inexpensive PSVane tubes so there is a strong incentive to upgrade.  Given the low cost it is not crazy to step up to EML or even KRs.  

I am sure there are others.  These are ones I am thinking of off hand. 

As a dealer I don't carry Line Magnetic.  I can talk to Eunice about KR for you to try and get closer to your price range with with Audion but I am a KR Tube re-seller and don't carry there amps officially.  It would be at KRs discretion.  I could also refer your to one or their re-sellers.  I am talking to opera-consonance about becoming a dealer.  At this time there isn't a re-seller in the US so I am pretty sure they will sell to me.  I am the importer for Art Audio so I have complete control over that line.

I understand completely.  When you hear something that you love, you want nothing else.  Also get that everything in Europe is more due to VAT.  

I couldn't find an impedance measure on those speakers either but generally speaking, Tannoy's are generally pretty tube friendly and I know they are brilliant with the Art Audio amps.  

This is interesting though, I just did the calcs for your reference ATCs.  If the ATCs are 85dB and receiving 35w of power and this was plenty for your needs, that puts your peak output at 98.7dB in normal listening conditions.  Listening position 2.5 meters from the speakers and the speakers are near the wall (~1M) but not in the corners. 

In comparison, the Audion EL34 SET is 12w into a speaker with an 89dB SPL, it is 98.1 dB of output.  It is a touch low compared to ideal scenarios but .6dB in output is not a lot. 

I was just running a speaker with an 83dB SPL at an audio show with a 45w amp.  I could still hit reference levels and it did sound good.  Now, don't get me wrong, I was at close to max volume and my speakers were near the corners of the room which helps enhance the sound.  But it would get loud in a 4 meter by 7 meter room.  Way above normal listening levels.  

What does the return policy look like on the Audion?  If you really like it...might be worth rolling the dice.  Have you heard it driving your speakers?  I would say with confidence 8w is not enough, but 12w is close.    Do you mind having the volume nob turned up above 50% most of the time you are listening?   There may be moments that you won't love, but 99% of the time you should be good.