Suggestion used tube amp between $1,500-2,000

For $2k, you can pick up a used Music Reference RM-200 (the original, not the Mk.2). It is well-suited for passive pre-amp use (input sensitivity, gain), so you can dispense with the LS-7 and get a better-sounding passive if you wish. Over twice the power of the good Quicksilver suggestion from yogiboy---100w at 8 ohms from only a pair of KT-88's per channel. And unlike any other tube amp with which I am familiar, more power into 4 ohms! 

I noticed that in your other recent thread you indicated that at an early stage of their breakin the new SF speakers have been sounding bright. Presumably that will improve as more hours are accumulated. But I want to caution that a tube amp, somewhat counter-intuitively, may not be helpful in that regard, and depending on the particular tube amp may actually increase the brightness.

The reason I say that is that the impedance of your speakers, as shown here, varies from less than 4 ohms in the mid-bass region to very high values, more than 20 ohms, in much of the brightness region (e.g., 2 to 5 kHz). The relatively high output impedance of most tube amps (compared to nearly all solid state amps) will interact with that kind of impedance variation in a manner that will give greater emphasis to the frequencies at which the speaker impedance is high. The degree of that effect will depend on the output impedance of the particular tube amp.

The RM-200 which bdp24 suggested has significantly lower output impedance than most tube amps (see Stereophile’s measurements), especially on its 4 ohm and 1 ohm taps, and so in that respect, at least, it would seem like a good choice. I’m not so sure about the other amps that have been suggested. However the 15.4K input impedance of the RM-200 is less than the 20K minimum load that is recommended for your ARC preamp.

I’ll mention also that nearly all solid state amps have output impedances that are near zero, and so with your speakers would not emphasize the brightness region as a result of the effect I described above. However many solid state amps will intrinsically tend toward brightness, compared to many tube amps, in part due to their tendency to incorporate greater amounts of feedback than many tube amps. So it may be matter of picking your poison.

I don’t have any particular suggestions to offer that are within the price range you indicated, but those are some thoughts I would consider. And before making any decisions about amplification it would probably make sense to wait until the breakin process of the speakers is further along.

Good luck. Regards,
-- Al

I would second the VAC 160. I recently heard this amp in a head to head with Aestetix amp and it was much more pleasing tho my ears. 

Where can one get a Vac 160 for $1.5-$2k? 

Another vote for Quicksilver and there is always Rogue Audio to consider in this price range.  I do disagree slightly with people recommending higher power tube amps though.  That will depend on your room size.  My speakers are 90 db sensitive in a 14' x 19' room and I rarely exceed 5 wpc, it gets uncomfortably loud at that level.   I know this because I keep old amps with power meters around.  So what is the room size and how far is your listening position?