A question about the SCA35. I assume the SCA 35 does not have 4 ohm speaker taps because 4 ohm speakers were not common in the early 60's. Other than an impeadance miss-match is the output section strong enough to drive loads rated at 4 ohms?
You may find the SCA-35 delivering its full power of about 15 watts/channel to a 4Ω loudspeaker because that nominal 4Ω rating is usually a MINIMUM impedance. Across most of the audio band the "4Ω" loudspeaker will have a higher impedance than that. Some vented loudspeakers rated as "4Ω" are at or over 8Ω everyplace but the "saddle" in the impedance curve.