Many people are afraid by the direct coupling of the speaker to the tubes, but you also find this on most of solid-state amps, and nobody fears though !
It is correct that most (I would say all contemporary) solid state amps don't have output transformers, but generally speaking, the voltages involved in solid state amps are low, and there is no reasonable failure of a SS amp that is going to put lethal voltages on the speaker terminals (or any place else that a user may come in contact with).
Even very high power SS amps have rail voltages at most around half of the line voltage, and these usually contain speaker protection circuitry that protect not only speakers -- but also listeners -- should there be a failure.
Even without speaker protection circuitry (or assuming multiple failures), comparing a 1000W professional stage amplifier to a 10W amp (or pre-amp) is not really fair. In a fair comparison, there is no SS 10W amp that is going to present significant voltage on the output terminals under any reasonable/imaginable failure modes.
That said, There are safe and unsafe Tube OTL designs.
OTL Tube amps that essentially just substitute a capacitor for the OPT (speaker connection on the plate side, conventional power configuration), are unsafe. Capacitors very often (I'll even say usually) fail shorted (or very leaky). Classic OTL designs seldom employed speaker protection. This means that a
single and common failure puts lethal voltages on the speaker terminals/speaker wires, etc. This is by definition (not by what I say) -- unsafe.
There are safe (or safer) OTL designs.
--- If the speakers (outputs) are connected to the cathode side (again, other things being of a conventional design), a single failure that would put High Voltage on the speaker terminal is unlikely. Most failures would either short out the power supply, blow the fuse, or just make the tube stop conducting (open).
-- If the speakers are connected to the anode, but the cathode is run at a large negative voltage. Failure modes are similar to above. Given a safe PS design, these
can be relatively safe.
-- Dual rail designs are a combination of the above, and work similarly to a modern SS amp. The speaker terminals are kept near 0 volts without an output coupling capacitor, and there is no single point failure that could allow high voltages on the speaker terminals (for more than an instant). These are again -- pretty much as safe to the user as a SS amp. I believe the amps shown in the previous post work this way.
--- There are also low-voltage tube designs that would not employ high voltages at all. These would be pre-amps, headphone amps, or very low power designs. (Tube car radios? and yes, there were tube hearing aids.)
In case anyone is wondering: Conventional tube amps (with Output Transformers) also have only a single component between the Plate Supply voltage and the speaker terminals -- this component, of course, being the Output transformer itself. So what's different than amps that do the same thing with a capacitor? After all, transformers
can and do fail shorted.
If a short develops in a transformer, most failures will also result (one way or another) in a short to ground. With the secondary (negative speaker connection) referenced to ground, and the transformer core/mounting firmly attached to the chassis, a failure that would present HV on the secondary without something else happening that shuts down the HV is very unlikely.
is it true that OTL adds the least amount of "tube color" to sound?
The Output transformer is just as critical to the sound of a Tube Amplifier as the specific tubes used or the rest of the design. An OTL amp is going to sound different than a conventional (with OPT) tube amplifier. An OTL design will still have softer clipping than a SS amp, and perhaps the "pleasant distortion" qualities of a tube amp, as opposed to "harsh distortion" of a SS amp.