Strength doesn't disappear, but it fluctuates on a cycle and takes nosedives sometimes if you take a break. For example, I sprained my ankle badly and didn't go anywhere near a gym for two weeks. I go back in and my legs cramp painfully squatting 135, and I couldn't put up more than 5x175 for a while. Within 7 days, I was back up to doubles at 315lbs... and the ankle's still not healed.
Training to failure is simply training to fail. Not recommended. Nobody that's strong does this, I guarantee it.
As a loose rule of thumb, steroids shouldn't even be a consideration until you've been lifting seriously for 10+ years and your lifts haven't budged in two years. Let's not pretend that "test boosters" aren't in the same boat.
I think you're confused.
Firstly, you're not toned; you have a low bodyfat %. The only people that say "tone" (and "core") are aerobics instructors because they preach to the weak-minded, -willed, and -bodied.
Second, you're trying to gain muscle without gaining mass? Don't say that because it doesn't make any sense. If you're trying to gain muscle, which has mass, you're gonna want to look to get a bare minimum of 300g protein a day. I'm willing to bet you barely get half that since most people barely eat any protein at all. If you're lifting 4-7 times a week, you're probably looking to intake something like 3,000-5,000 calories a day to keep getting stronger. This of course, depends on your body and how much and hard you work. Basically, if you hurt and feel weak, eat and sleep more (and attack the weights like like a rabid wolverine). Nobody is packing on muscle with 2,000 calories and day.
As an aside, clean bulking is SLOW bulking, which is why powerlifters and bodybuilders cycle their diets (and lifting routines) to gain quick mass and strength, then diet back down for competition. There's nothing wrong with taking it slow and steady, since you're in the game for the rest of your hopefully long life, but for someone that's so frustrated by their current plateaus that they're considering steroids, I can't imagine you're looking for slow results.